The new terrain editor does allow you to create custom plane skins but only as part of the terrain, if that makes any sense.
I think allowing everyone to be able to make and then upload their skins to the AH servers would be ripe for abuses. You'd see Rainbow colored Spitfire skins, Pink with lace trimming La7s, Rose colored N1K2s and the like. Not a pretty sight.
Ack-Ack