Make GM/event exclusive headgears become costumes (by default)
Make all exclusive headgears, i.e. event prices costumes by default and retroactively. There are rare headgears like Poporing Cap or Baseball Cap which are no costumes.
Event exclusive prices like the headgears should be commonly available as costume headgears as you have to sacrafice a precious upper headgear slot (for e.g. Poopoo during WoE) if you want to show it off.
This issue does not necessarily have to effect the Costume-NPC's list, but if it's no big deal you could have a look into the database how many and which of those "exclusive" headgears exist and add them to the NPC.