I've heard of bananas being put in the fermenter or keg for hefeweizens, but, in my opinion, it's unnecessary. The yeast should give you the banana/clove/bubble gum flavors on it's own. I would worry that adding banana may be TOO much for the beer or getting a weird off flavor from the boiled banana.
Of course, it's your beer, and you can do whatever you'd like. Experimentation is one of the fun parts of homebrewing.