There actually is a plan for the Christmas and Halloween Title Screen outfits to be playable. No idea when, though.
Well, this is awkward. I was close to completion before reading this. Had I known this earlier, I would have moved onto another poke-cg.
Nonetheless, here is the set of assets for the xmas costume, though as modding costumes has not been implemented, these assets will replace one of the original.
The wardrobe replaced will be index 3 for the hat and index 1 for the rest. Make a backup copy if you want to change back later. Copy the folders in the zip into 'ROOT/Graphics', where ROOT is where Game.exe is.
As a bonus, I've included hat and shoes for pole-dancing, missionary and reverse cowgirl overworlds. To add them, in the zip, open, for example, 'Character/wadrobe/allie/shoes1_lap_allie.png' in an image editor. Copy the whole image, open 'ROOT/Graphics/Pictures/H/overworld/f_lap_allie.png', paste and save.
The original pole-dancing sprite is at 'ROOT/Graphics/Characters'.