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!