I am a member of the Spelljammer group on Cartographers' Guild. The group got set up to help people find Spelljammer maps posted on the forums. Sadly, it seems that not many people are interested in making maps of asteroids for Spelljammer.

Would anyone want to map a Spelljammer asteroid? It would be very similar to an island, but floating in space (rather than surrounded by sea). And it could be an asteroid-city, a baren wilderness, covered in farmland or any other funky combination you can think of.