One thing to possibly try is to enable extended profiles, disable user profile uploads and see if you can change the avatar yourself from the users profile page. Might not be workable for you but that would be my first approach, as with anythin it’s best if the functionality is already build.
For what you are asking you’d basically need to write your own plugin that redirected the BP Avatar load to the bbPress Avatar. There’s nothing that does that as far as I’m aware and it normally works the other way round – the BP Avatar is used for both when both are installed.
