Considering the character customization aspect of sun and moon, I think it would be pretty difficult to swap character models with an npc unlike in oras where the player character (having no customization) was just a single model. Perhaps if you were to change one item to Wally's model and somehow make the game not load the other models or replace them with blank ones? I've tried fiddling with some model swaps before, but it was mostly with other customization items, and they very often crashed my game. So if you can get it to work without crashing kudos to you.