“Whomever” is correct, but not exactly for the reason you state. It agrees with the subject of the dependent clause it introduces.
So:
Welcome back to whomever this is a sock of.
(This is a sock of him)
But:
Welcome back to whoever brought the pie.
(He brought the pie.)
I’m trying to think of some examples. I think we’ve had a few that were racially oriented, and attacked the board for not supporting non-whites more vehemently. I’m guessing that LW trolls visit RW boards, overall. More “food” to be found.
I forced my way through that thread. I vote troll.
The trolldar is strong in this one! 
Nice to be vindicated on this issue, thanks. Now if you could re-frame the sentence so that it doesn’t end that way. “Welcome back to the user of whom this is a sock” is pretty awkward.
It’s okay to end a sentence with a preposition. There may be some cases where it might be problematic (I can’t think of one off-hand), but, for the most part, it’s just a grammar superstition not to end a sentence with a preposition and in the same pile of nonsense as not splitting infinitives.
Or, as Churchill did not say (but wishes he did), “This is the sort of arrant pedantry up with which I will not put.”
In truth, “whom” is a bastard child of fading grammatical morphology that honestly deserves to be binned once and for all. “Who” and “whoever” ought to be adequate for all uses, because, really, “whom” just sounds stilted anymore.
I’m pedantic, here and elsewhere. I will lift my nose, and sniff, at whom I choose.
I’m happy to employ the jolly old standby, ‘whichever wunna these fuckers’. It doesn’t always flow, but it does capture the attention of the listener.
If there is the welcoming back, I think it is to this this long dormant one who since the america elections after more than a decade in the toy box now is looking to make people dance with it.
Nah, whichever wunna these fuckers thinks it doesn’t flow is out of their little head. It’s one of those phrases which when delivered with the proper accent has a musical rhythm that cannot be denied. It’s similar to when, “Fuck you, motherfucker”, is delivered with the proper comedy K’s.
http://boards.straightdope.com/sdmb/showthread.php?t=820634
I did not see that coming. Glad it happened, but surprised that’s what pushed him over the edge.
well, when you’re getting beaten down with your own poor arguments and choices of words, you have to invent some supporters, apparently.
So was he The Dreams Expert, banned after 9 innocuous posts?
So who has the time and energy to do research on other recent bannings to determine who his socks were? I’m curious if these were recent creations or longer-term endeavors.
Were they LinusL, LinusM and LinusN?
If you have questions about unannounced bannings, PM a moderator. But LinusK’s sock was disappeared.
I don’t really want to know the answer; I prefer idle speculation. The truth is usually pretty boring.
I’d speculate the sock(s) were men’s rights wackadoodles.