For this, we rely on canned beans, and just about a quarter of their liquid, which adds a bit of extra flavor to the dish (though I agree that too much of the liquid can produce something a tad slimy).
Mashed bananas are similar in thickness to pureed blueberries so you most likely won't need to addextraliquid, you might want to add 2 tbs of the banana to make sure the brownies don't turn out dry though.