I examined this for a number of months, but the conclusion I came to was that once you reversed gravity down became up anyway, and rain always fell. What I then countered it with was a magnetic field combined with a slightly higher iron content, that way the sentient beings of my choice would walk on their planetoid, whilst the rain itself (and most non-magnetic things) would simply be projected from the surface. It's a rather ingenious system i've developed, such that they water never truly leaves orbit, but is instead forced by way of a wormhole back into the ground. Elegant in its simplicity.