Bananas are notoriously difficult to preserve without unpleasant textural, colour, and flavour changes occurring. In weizens, it is the yeast that provides the banana and clove notes. If you feel you must try it (and, hey, ain't that what homebrewing is for?), I'd suggest going with dried banana chips if you can find some without salt and preservatives. Flash-pasteurize them in a tiny bit of boiled water to 170*F for a minute or two or soak them in a bit of high-proof alcohol overnight (I'd probably go that route...the right selection could boost the spicy-clove notes too!).