I heard of a couple of boats training their meows to use the head!
It starts with using a round litter box, then gradually moving the litter box to the head floor. The next move is to use a round litter box to fit over the bowl, then a smaller litter box inside the bowl .... then .. ole !! the meow squats over the bowl to do it's business.
I don't think anyone's perfected the way to teach them to flush the bowl yet ...