I would say just get the word out that you are looking for customizers to submit their work to you for display at your gym.....hell, doesn't even have to be limited to minimates, have a shelf, section, own display for statues, larger scale figures. Find folks who would want to do murals, paintings or other types of wall art of different genres of retro to current characters from movies, comics or cartoons.
I, for one, wouldn't need an incentive..... just having my work on display and something showing where folks could see more would be incentive enough.