I tried the same thing as you for my second-ever brew a few months ago. I had frozen the bananas, blended them with 99 Bananas liquor to sterilize them, then added them to the primary after the initial fermentation had settled down (about 5 days, as I recall). The result was a higher-alcohol, and decent-tasting Hefe, though not specifically to style. But it ended up being much darker than I intended. I was wondering if the banana mush darkened (due to oxidization?) much like bananas do naturally when they ripen. Also, their was only the faintest hint of banana flavor.If I had to do it again, I would likely add 2 oz of natural banana extract at bottling. (I say 2 oz, because my next one was a pumpkin ale , and I felt that 4oz [for 5 gal] was a bit much). Bananas are really high in sugar, and I bet the yeast will mostly chew them into EtOH.If you've never had Wells Banana Bread Beer, go obtain some. As far as banana flavor, thats what I'd be shooting for. Great-tasting beer. FWIW, they say they use real bananas as well as banana flavoring, though I've no idea in what context.
"If you serve this beer to a Michelob Ultra drinker, he or she will cry." - from description of Northern Brewer's 115th Dream Imperial IPA extract kit.