one idea might be to have the prize pools combined. I've seen that in other events for things like highest scoring player U1800 or U1600. Highest scoring woman gets her place on the woman's prize pool and whatever amount they'd win in the Open

This can work in small events when the prize that lower sections compete for is a nice book or a few hundred dollars. Once you have final round games where for one of the players a win means winning a five digit sum while their opponent can only get a few rating points out of the game, possible match fixing gets too profitable.