Excellent idea!! How ripe do the bananas need to be to replace half the butter in a recipe? What if the recipe calls for oil...does it substitute as well?
When I have unsweetened applesauce, I use it instead of oil in cakes.
I use them in all stages of ripeness -- the riper they are, the better they are for you but also the stronger their flavor, so for delicate flavored cookies, I use barely ripe cookies to not offend some of my picky children, lol, but for stronger flavored cookies the bananas can be the healthier overripe ones. :) It's two-fer -- more fruit in our diet and also less fat!