Agreed. Disney wouldn't introduce rap into their movies so as you mentioned, they can always dub over Will's singing if he can't do so according to Disney's standards.
I wouldn't presume to say Disney wouldn't do one thing or another. There are firsts for everything. They were strictly an animation studio before doing 20,000 Leagues Under the Sea, their first live action movie. They never did a PG-rated film until The Black Hole. They held out on doing a film that was rated PG-13 until Pirates of the Caribbean: The Curse of the Black Pearl. (first PG-13 under the Disney name) So, they will occasionally break from their traditions and do things they've never done before.