What do you think about the idea of a costume category?

There is a costume category in CAS under full-body outfits! But I don't know how the game pulls from that category and/or others to make situational outfits for costume parties etc.
I wonder if one of the situational outfit mods out there would do what you want.
I've had themed parties before where I invited other sims I play rotationally, and I handled the themed outfit by making the other sims additional outfits in the party category ahead of time. Some sims showed up in those, and for others I was able to use testingcheats on and shift-click to dress them in the outfit I wanted.
It would have been easier at the time to just replace their party outfit with the themed outfit in CAS, so that they'd show up in the themed outfits by default, but I didn't want to go through the work later of reconstructing their original party outfits.