This may be a weak argument but I can see another reason why Sora can be in Brawl.
It doesn't involve Sora in the game but he was the basis of everything so far, and thanks to his success in Kingdom Hearts (I, CoM, II), there will be more KH games (as some of you may already know). There will be a PSP version and a DS version, and producers are working on fixing the system for the DS since it lacks several key buttons. And again, Sora is not in the game, but what better way to start marketing for the KH franchise than to include Sora in a mainstream Nintendo game? (They did this for Marth and Roy in Melee, and look how popular Fire Emblem became in the states later on. Sure heck got Ike's popularity up and earned him a spot in the cast.)
Surely, this will be a good move on both Nintendo and Sony's side, not to mention, Disney as well to some degree. I know Sora should not be used for pure marketing purposes, but this could be motivation for future KH games, especially for the 358/2 Days game.
And please, no more of this fanboy/anime references. In retrospect, if it applies to Sora, then it applies to Megaman, Bomberman, Pac-Man, Geno, and heck, the rest of Brawl's cast. Just because we support doesn't mean it will happen. Let us be and if you don't like here, go make an Anti-Sora thread (only for it to be closed) or go support someone else.
So maybe after my argument, Sora's chances went from slightly to a-little-more-than-slightly.
...hopefully...