I don't have conclusive answers, for you, Pokedude. I can tell you a few things, and I can tell you what I don't know.
I DO know that Join Avenue WILL allow you to get a stat raised after 100 EVs in that stat. I know this because I got its services for Pokemon after they rejected vitamins.
I don't know if it actually contributes to your EV total past 100, though I would assume it probably does, and I don't know if it'll stop you at 252, or allow you to keep paying.
Anyone done some careful research who can help us?