Two suggestions: 1) Try a box of tempura mix and use 1/2 water and 1/2 coconut milk and fry them to order. 2)The only way I can think of to work more in advance is to have your staff cut the bananas into thirds, soak them in coconut milk, roll in sugar (or cinnamon and sugar) and wrap them in fueille de brick pastry that's been brushed with egg so it keeps shape and then fry them to order. That way during the dinner rush, they won't have to batter dip them, they can just drop them in the fryer.