first of all, are builders (of the entries) allowed to judge?
I think the best way to get judges would be to have advertisement on the ingame welcome screens (on the 3 games) & to try to get some persons that you know & play with. The only problem is that they need to have dled the 3 games (stadium/canyon/valley) which is possible without paying but still takes time.
So yes , the solution would be advertisement in game (link in your name to the judge topic, etc).
e/ another good thing would be to convert the bunch of ppl that still didnt move from tmnf to tm², a lot of oldies could bring much more activity & challenge to those contests. (for exemple we can tell them how better the editor is, i.e. the copy/paste tool, bug-free integrated blockmix, newblocks, embeddables objects, macroblock ...)