If the problem with this dress is that it ONLY works with that belt because of the gap under the belt, then the solution is painfully simple. Make this a special category like when they added Togas and make it so that belt is automatically equipped with no other options (including no None option). The only option from the player's perspective is how to color it.
After that, if players want the flowy arm things... shoulder option, or maybe they might want something else in place of that. Bikini or shorts or full leggings, whatever tights or bottoms with skin options the player wants to customize with, same for the upper body under the dress. Limit the belt and leave the rest up to players' to play with.