Figured it out. Easy fix: just pick a folder that comes after opponents_core in the alphabet.
The code reads through the folders alphabetically looking for opponents. If it finds a folder with outfits but no player.properties in it, it doesn't know who that outfit is for yet, and just drops it. You can also put a copy of the opponents.properties and portrait.jpg files in there. That fixes it too...
... except if that player's name is Vivian, because there are a couple terrible bugs in the "shared outfits loader" part of the code. Those will flat out crash the game if it finds Vivian and her cheerleaders outfit before it finds Candy. Or in other cases give Candy the outfit twice. Those will be fixed in the next release though.