How did this particular dish become part of Hawaiian cuisine? Your basic lunch plate will have meat, a scoop of rice, and a scoop of mac salad. the first two I get. But mac salad? Did the missionaries hand out elbow macaroni with the bibles?
The last time I looked into it the consensus seems to be that nobody really knows.
The plantation owners could have introduced it (possibly starting with potato salad but it getting switched to macaroni salad because the ingredients were much cheaper for workers to make at home).
Others say it came in via hotel chefs serving mainland clienteles.
But apparently it is confirmed to have been part of a standard plate lunch as far back as the 1920s.
Ohana, in Belltown, has good hawaiian macaroni-and-potato salad. Since I can’t (reasonably) get there anymore, I want to make my own. Just haven’t had the time.
Oh Thank Og! I thought there was going to be pineapple and Spam™ in it. Whew!!!
Like Spam, macaroni salad could well have been introduced by the US military when it moved in after 1898.
When I was stationed in PH, mac salad was a staple at the various outdoor potlucks we attended. Often, in addition to a couple of cubed potatoes, there would have been a can of tuna mixed in.
Never at Rainbow Drive-in, though. It seemed to have been a home-cooking-only innovation.
To build on this, here is the Wikipedia entry about the plate lunch. And the local newspaper, the Star-Bulletin, ran a story on this in 1999.
(Personally, I order all rice. Not a fan.)
I’ve tried a half a dozen times now to make macaroni salad as good as what they serve in Hawaii. So far, it always comes out too dry, no matter how much mayo I use. Since I don’t want a salad that’s nine parts mayo to one part macaroni, I stopped trying. That’s not a bad thing; it’s probably one of the most unhealthy things you can eat. Good, though.
If it’s proper mac salad I go the other way and just get that with no rice (with some diet restrictions a proper plate lunch is going to last me 2-3 meals so that keeps better than refrigerating rice).
Where I live in the San Francisco area I have two walking distance plate lunch places.
One has good chicken katsu curry (my preferred) but served mediocre potato salad for some reason with it.
The other has mediocre chicken katsu curry but fantastic macaroni salad.
I’m always very torn which to go with.
Screw it. Get the loco moko. 
Moco is spelled with a C. 
But I’m glad you didn’t say moco loco.
This calls for a dastardly scheme to either get the good katsu guy employed at the good mac salad place, or the good mac salad guy employed at the potato salad place. Do you have any ACME products handy?
Tuna in mac salad is something I’ve also only seen at potlucks. I also saw a variation that had curry powder in it, giving the dish a yellow glow. Can’t say how that one tasted, as, you know, eww gross.
I have yet to come across a really good loco moco. They’re always pretty bland. Of course I’ve never tried one in Hawaii.
I sometimes make shrimp cocktail with mayo, curry powder, and dashes of lime and Tabasco. It’s delicious! Haven’t tried it with mac yet, but I imagine it’d be good too.
I have heard that you can only use “Best Foods” mayo - full fare (not “Lite” or lower fat) in order to get the proper texture. I can vouch that when we use Best Foods mayo it does come out about right.
Re: the OP, no idea. Perhaps they tried Mac-n-Cheese and found out the locals were lactose-intolerant, so had to figure out some way to use all that pasta.
I don’t know if it’s authentically Hawaiian but this recipe is great. I’ve made it several times for family dinners and potlucks and every one seems to like it. It is definitely not for calorie counters.
In Salem, there’s a Filipino family that run a terrific island bbq joint. The specialty is katsu moco, which is just what it sounds like, and while I would not have tought of it myself, the combination is amazing.
No peas? No potatoes?
I don’t like the photo, but this recipe looks more like what I’ve had. I don’t recall any curry powder, though.
The second best restaurant loco moco I’ve had and best outside of Hawaii is from King’s Hawaiian, in Torrance, CA. (King’s started in Hawaii in the 1950s.) So if you’re ever down here again, give them a shot.
It’s a bit far from me, though, so I just make loco moco at home. The keys are well-seasoned, no-filler beef patties, and dark, salty brown gravy. A bland loco moco is one that was made with restraint, and restraint don’t belong here. 
My mom makes a mix of imitation crab, mayo, and curry powder and tops celery ribs with it. I can totally see how creamy curry can be delicious!
That recipe is pretty close to mine (that I’ve tried to make taste like the macaroni salad the lunch ladies at Kalakau’a Intermediate School made). One thing: try about 1/4 - 1/2 cup finely grated and soaked white onion. Soaking removes some of the strong onion overtone but still has flavor. Use the scallions to taste and save out some for a bit of garnish crunch.
Here’s the recipe from the L & L Drive-in (now a chain but the original was in Waimanalo):
http://www.food.com/recipe/macaroni-salad-from-l-l-hawaiian-bbq-498225