Well, I don't have alot of experiance but I wouldn't think banannas would ferment very well and if they did yield a tasty flavor- I'm thinking it would be hard to filter and even clean. Kinda similiar to using pumpkin. That siad, try it and let us know how it goes!
The other thought is to use bananna extract - I had it in my hand this evening at a better supermarket where I live thinking that maybe it would work as a soda flavoring. Didn't think about using it in cider! In fact, check the baby food isle - don't they make that apple/bannana as a drink? I'd look on the ingredients to see what's in it - probably an extract. The Organic isle might have an all natural extract as well- I didn't look hard at the extract ingredients that I had. BTW, I didn't get the extract- ended up getting strawberry for the soda!
Bananas are useful in wines to give body without alot of flavor. I guess that means that you'd need alot of bananas to get any flavor out of them. They make a wine kind of "thicker" so thin bodied wines really benefit.
Banana wine is actually pretty good, so a banana cider might be ok.
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Somebody smack me if this is a crazy idea, but what about using a hefe strain of yeast to ferment the cider? they typically give a "banana" aroma or flavor right? Just a thought.
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Bottled: Red ESB
Drinking: Ginger Kombucha
On deck: Rye Kvass experimental brew, Apple Cider Vinegar