As of time of writing, Costume Accessories are classified in the Auction House as Decoration Accessories (Deco. Acc. under Decorations.) Here, they are all mixed up:
Head Accessories
Middle Face Accessories
Lower Face Accessories
Back Accessories
These items being mixed up makes it difficult and inconvenient to check for a specific part. Now that we have a feature that allows us to preview the item, it makes more sense to organize the AH and keep the items separated.
Having them separate actually helps a lot, especially for canvassing, as well as determining prices for items that don't frequent the AH too often.
The large number of Drake hats doesn't help, either.
No idea if there's actually an item that utilizes the Costume Pants(?) slot, but if there are, they should be displayed apart from Costume Tops too. (under Deco. Clothes)