I personally don't think it'd work too well. I'm SOOOO gonna get flamed for this though, aren't I?
Simply put, guys and girls ARE different. Guys tend to be more logical, focus more on their thoughts and using their mind, while girls are more emotional and focus more on their feelings (and the feelings of others).
Now, I'm not saying guys are better than girls or vice/versa, I'm saying that, for the main character of KH (Or similar action RPG games), I think a guy is a better choice. I mean, KH is supposed to be a game about adventuring and action, going and being the warrior, the woman's natural role is not the warrior, but the comforter and nurterer. Now, the roles can be reversed, but it simply doesn't work as well. Women are also BETTER comforters and nurterers on average than men, and men are on average better warriors and hunters/gatherers. If this were a game about raising kids or simply helping people, maybe, but it involves QUITE a bit of fighting, and killing. I simply don't think it'd work as well.
On another note, there are still more guy gamers than girl, and I'd like a main character that I can kinda rewlate to (being a guy).