Here's an idea: Instead of waiting for costume creators to come to us and submit on their own, why not take the initiative and ask the artists behind your favorite retextured costumes for their permission to use their work in Minus? I bet there are plenty of awesome costumes (and stages, even music...) out there already, which the owners of would gladly allow us to use officially, if we would just ask -- but they might not even know that Minus exists!
You can't expect all of the new content to just come to us without us doing anything to seek it out. Case in point, we would have lost the taunt = footstool feature (because our version desynced online) if I had not went to Magus and ds22 and asked for their help.
This is also mentioned in the OP and allowed. If there is permission for an existing costume, with proof from the creator, then we will let it enter the same way any other costume has. I've done this for a few of the current entrants, but feel it would be more significant if they were costumes created or requested by members of the community and the BR just decided among those.