What’s wrong with opening 3D as a pre-empt? You don’t need any points to do that. Of course, it does rather give away your shape and (lack of) points, but obviously the point is to use up bidding space. If LHO now wants to start with clubs he has to do so at the 4-level, which probably makes defending a spade contract the most likely outcome - one you would seem to have a reasonable chance of defeating, even without knowing the other hands. The only problems seem to be that you might miss a spade fit yourself, but if partner has the points to bid on over 3D, you can probably end up in spades. On partner’s hand as dealt, he will presumably be itching to switch to hearts and it probably doesn’t matter too much if he does or not, as long as (like you said) he is sensible enough to shut up as soon as the misfit becomes obvious.
The danger of missing a spade contract is too great. If you open 3D there is no way you will find a 4-4 spade fit. Partner will only bid a 5-card suit if it is very good, and your having the ace makes that unlikely.
I am not totally averse to preempting with a 4-card outside major, but this hand has two flaws: bad diamonds and good spades. I would open 5432 x KQJxxxx x with 3D.
Fair enough, that makes sense - thanks.
Preempt with a hand containing 1 flaw, but not 2 (or more), is a good bridge maxim IME. If you choose to only bid very pure wk 2s or 3s that is playable and great for constructive / competitive auctions, as partner knows where they stand - but it’s unacceptably passive for disrupting their auction, you need to open it out a bit. The hand posted actually has three flaws - terrible suit quality, outside 4cM, and a void - doubt you’d get many good players going 3 with that.
Similar with weak jump overcalls - sticking your oar in with an horrific suit is OK if pard knows what to expect. If they take the save in 5m expecting 1 or 2 off and it’s more like 3 or 4 then you’ll get hammered in club bridge, because ‘keeping them out of slam’ won’t apply to most tables.
My tournament experience is almost a half-century in the past, but I’m not certain I can fully endorse this.
You have 8 diamonds, not just 7. And their very weakness calls out for them to be trump!
If partner has Qxxx - xxxx - Ax - AKx (a hand I wrote down almost at random, NOT cherrypicking), ten tricks in diamonds are probable, but eight tricks in spades may be difficult. Even if partner has four spades and a diamond void, you’ll probably have at least as many tricks with diamonds trump as with spades trump, no?
Besides, won’t 3D 3H P 4H 4S show an 8-4 ? 
Here’s a distributional hand that just came up in a pick-up game. We played Four Spades.
♠ A853
♥ 2
♦ 76
♣ KQ8742
♠ - ♠ K764
♥ KT9754 ♥ AJ86
♦ QJT985 ♦ -
♣ 5 ♣ JT963
♠ QJT92
♥ Q3
♦ AK432
♣ A
With N-S vulnerable, West deals. Compared with some tables, our auction was very tame: P P P 1S 2H 4S P P P.
The Diamond Queen was led and ruffed in East. Back came a small Club to the Ace; Spade Queen was led and overtaken with Ace. Then two Clubs were cashed, pitching hearts.
♠ 853
♥ 2
♦ 7
♣ 874
♠ - ♠ K7
♥ KT97 ♥ AJ86
♦ JT98 ♦ -
♣ - ♣ JT
♠ JT92
♥ -
♦ AK43
♣ -
The lead is in dummy. You need six more tricks (though a 7th trick might be nice for an extra imp fraction.)
How do you play?
(I do NOT think this is difficult. I’m posting partly to check my sanity.)
To save everybody time, I think you get six more tricks almost regardless of what you do. It just seemed like an interesting freak, especially looking at the wild auctions at various tables. I led a diamond from the dummy at trick 6, letting East waste his small trump on my diamond loser. That seemed the most straightforward way to get the hand over with.
What made the hand somewhat memorable was that my pick-up partner began cursing me, accusing me of miscounting trumps. :smack:
Help! Please join us over at PlayOK.com. I have only one real partner there — an excellent card player who, like me, played tournaments back in the Jurassic Era. And he only has me. Things get horrible when one of us takes a week off, as he just did for Thanksgiving. Won’t one of you Dopers log in and join us? 
Do you play on BBO septimus? That is the centre of gravity of online bridge, largest player base, and gives you the most options for a game that suits you. There’s a fair amount of wading though idiots to be done in the first instance as you figure out the site, but it’s great once you’re familiar with things. I tend to use it more for card play practice [robot games] nowadays, but there are loads of options for a good human game.
If you’re comfy with another site then obv you’d want to continue with it, but if you’re saying it’s tough to get a good partner then BBO is worth a look.
I tried BBO, but found it difficult even to log-in — perhaps its interface couldn’t tolerate my sluggish Internet. Once in, I found I preferred the elegance of the PlayOK interface enormously.
As just one example, consider looking at the last trick. On PlayOK the previous trick is available for view in a corner of the screen until the current trick is complete. IOW, you can get distracted after playing a card, knowing the trick will remain on view until after you play to the next trick. On BBO, IIRC, there’s some unnecessary clicking protocol to view or fold the just-completed trick.
I do have the one regular partner at PlayOK; we play enough to satisfy each other’s needs! 
I just thought it might be fun having a trio of Americans bidding à la American: we’d have a “spare” player! Most of the PlayOK’ers use the Polish Club system (if they have any system at all).
How would you open SAQJTxxx HAK DAxx Cx? My partner opened 1S on this. I, having a singleton SK and nothing else passed. Of course 4S is cold. I did consider bidding 1N and passing whatever rebid partner made but we were playing 5-card majors so I knew we had a least a 5-1 fit. When I saw the hand I suggested she should have opened 2C, the bidding going 2C-2D-2S-2N-4S. She objected because she only had 18 points; I pointed out that she had a game-going hand. How say you?
Points, schmoints. 2C all the way. 9 playing tricks in a major in a strong hand, as opposed to a preemptive hand, is a 2C opening. This assumes you are playing weak 2s. Playing Acol 2s, opening 2S is fine.
Yes, we were play 3x Weak Twos.
How would you open SAQJTxxx HAK DAxx Cx?
Assuming you play weak twos, this is a hand to open 2 Clubs on. After that it depends how you play responses. If 2Ds is weak showing a K (or perhaps 2Ks) or less, jump to 4 S.
If you happen to play control responses (There are 5 possible 3K’s and an ace.) If partner shows 3 you very likely want to be in slam you have at most two losers.
2D would show less than AK.
This kind of hand is on the cusp.
Yes, it’s a strong playing hand for spades, only 4 losers in a self-sufficient suit that can expect to lose only one trick opposite a void, and thus one trick short of game in hand, which meets one common definition of a strong 2C opener: “would you feel sick if partner passed your opening 1 bid?”
On the other hand, as you say, it only has 18 HCPs and not a lot of defense. If you open 2C and it goes 2NT for the minors on your left, pass by partner, and 5C on your right, should you bid 5S? But if you don’t, how can partner evaluate what kind of hand you have?
I would prefer to open “all but one in hand” hands in a single suited major with a 4 level opening bid. To distinguish the more purely preemptive type 4 bids to avoid missing slams, you can play Namyats - open 4C and 4D for the highly constructive “8.5 to 9.5 playing tricks” major suit hands in hearts and spades respectively, leaving the 4H and 4S as purely preemptive, and the 2C openers for the true Game In Hand No Question type hands.
BTW absent a Namyats agreement I would prefer to shoot out 4S there than 2C. You might miss some close slams but you won’t miss a game, and guess what’s more likely? And you can’t get into trouble with partner assuming you have a ton of HCPs. Bid where you live. Or as I would say at the table, “Me caveman. See game, bid game.”
I think that if you’re playing 3 weak 2s and no other system (like Namyats) to show a strong single-suited hand you have to open hands like this 2C, otherwise the risk of getting passed out is too high. Effectively, the 2C opening becomes “a hand that can make game opposite a hand that would pass a one-level opening”. You do need to make sure partner is aware of this, otherwise sooner or later he’ll be putting you in a doomed 6NT.
4S might work in the short run, but if you’d also open 4S on (say) 8 Spades to the AK and nothing else, partner will never know whether to stick or twist when someone competes over your 4-bid. Or what strength he needs to investigate slam.
Or you could play something like Namyats or Benj-Acol, have a separate bid for the 8-9 playing tricks single-suiter and keep 2C for the genuine 23+ HCP rock crushers. But then you give up at least one of your weak pre-empts. Swings, meet roundabouts 
That’s what I meant by it being better to miss some corner case slams than corner case games. These “just one trick short of game” hands should be treated as “not going to stop below game” - I think we’re in agreement - but better to overload a 4S opener on the strong side and miss a possible slam when partner has the right fitting cards, than to open 2C when you don’t actually have game in hand.
That’s my opinion, based on 25+ years of playing, you can choose a different style, of course. But picture partner holding
K
xxxx
xxxx
Axxx
If you open 2C and opponents overcall 2NT for the minors, this hand should double to show interest in penalty doubling at least one of the minor suits.
Now over 3C, 4C or even 5C, if the 2C opener rebids 4S or 5S showing the strong single suited spade hand, this hand can see 10+2 tricks and go to slam… But not if you’d open 2C on 9 tricks.
AQJTxxx
AK
Axx
x
Isn’t good enough, but make an x of diamonds the King of diamonds or the Queen of Hearts and you’ve got a fine 2C opener.
TL,DR version would boil it down like this:
Open 2C on tricks in a single suit in you have game in hand.
Open 1 of the suit otherwise, unless you’re too good to open on the 1 level and can’t stand it getting passed out.
If 2 and 3 level bids are weak, and you have no systemic way to distinguish them, you’re left with opening 4 on one to 1.5 more winners than your minimum… Or opening 2C with less than game.
When would opening 2C light ever gain you, versus when would opening 4S heavy hurt you?
You don’t need game in your own hand to open 2C. See Larry Cohen’s view:
That is not even as good as the hand we have been discussing.