How does one alter a recipe that calls for one fruit with another fruit of differing water content? For example, I often make banana bread. I'd like to effectively keep the same recipe but use pumpkin instead. Bananas are ~72% water while pumpkins are ~90% water, and roughly a 20% increase in water content seems substantial. (I actually have a loaf of apple-bread in the oven; and apples are about 80% water—we'll see how it goes. :D)

I wouldn't be surprised if there were other things I'm not taking into consideration (e.g. - chemical composition of fruits) that affect any substitutions I make. This being said, is there some heuristic I can use when substituting fruits in baking? Or are the differences between fruits enough to warrant totally different approaches when going from fruit to fruit?
There's no trick, there's math. 100g of banana is about 75g of water (1g of water is 1ml, so easy to measure), 12g of sugar, and 13g of fiber and other stuff. A pumpkin is about 92g of water and 3 grams of sugar, leaving 5g of other stuff. 

My banana bread recipe calls for 2 medium bananas, that's about 250g of banana. That's 188g of water and about 30g sugar. If you add 250g of pumpkin instead that's 230g of water and about 8g of sugar, so pumpkin would add 42g of water (42ml, a bit less than 1/4 cup), and take 22g of sugar. That's enough of a difference to really throw your recipe out, so you'd want to remove 42ml of water, milk or other liquid and add 22g of sugar (4g of sugar is about 1 tsp, so 22g is about 5.5 tsp, or just under 2 tbsp).