Add a bit to the monst struct that tracks whether the monster is hiding on the ceiling or not. (Only valid for things like wumpuses, piercers, and perhaps bats but not actually since bats don’t roost on the ceiling when inactive.)

Then, when polymorphed into a wumpus or piercer, you can use < when not on stairs to climb onto the ceiling and > to climb back onto the floor, but only when adjacent to a wall.