If the temps drop too low (as per the yeast strain used) you could run the risk of stalling the fermentation as the yeast go dormant or sluggish and don't finish the job. As stated above, bananas have a really thick peel on them and I too think you would be fine just mushing them up right out of the peel, adding them to your fermenter, and racking the beer on top.
EDIT - I have never brewed with bananas before, and after thinking about it, they are quite starchy and I wonder how well they would ferment out. Maybe they would be better mashed with grains to help them convert?