Please refer to our answer about substituting fruit for oil in baking, but remember that bananas have a much stronger and more distinctive flavor. We wouldn’t use bananas as a substitute unless we were making something that was meant to taste like bananas. We have heard of one person, a vegan, who substituted a banana for every stick of butter in any baking recipe. Our source told us that the results were edible, but not very good.
a little aside, one of our writers makes a creamy dish with chicken livers that he serves over rice. He often adds a dash of apple or white grape juice to the sauce near the end, as it needs a touch of sweetness. Apparently one day, he reached into the refrigerator for juice, poured a couple of tablespoons into the sauce, and only then noticed that it was a hybrid apple-strawberry-banana juice. He hoped his family members wouldn’t notice. They did.