What if you gave away a free month of game play to addon makers for each "unique and functional" addon they submit to Trion? In other words, set up a system where addon makers can submit addons to Trion, Trion can look them over and test them to make sure they are "unique" (you can't submit the same DPS meter in different shades over and over) and if the addon is deemed worthy the account is credited with a free month. That way you can entice more people to build addons, and with addons you can entice more people who just want to be able to modify their UI to come play as well.
Seems like a win win. (I know the amount of submissions could possibly become a lot, but I don't think that as of right now it would be too big of an issue.)