Afaik that can now be achieved quite easily with the clothing_override method.Sorry for yet another suggestion so quickly, and maybe one that'll be too much trouble/too little return...
I was thinking about 'special' characters who use static images (at least I've used a few); maybe there's a better method, but I've been using alternate cultures for them. But what if we could have a 'set_occluded' command? It could take a yes/no state for add/remove, and a GFX name for the image to be applied?
Code:some_character_scope = { set_occluded = { state = yes value = "GFX_My_Occlusion_GFX" } }
That mechanic is pretty powerful and can be used creatively - I know because I even managed to override child portraits with static gfx, while vanilla children portraits were never meant to be affected by this and don't even use a clothing layer.
So I suggest you try to use that method instead of custom ethnicities. Your suggestion would of course be even more convenient, but imo it's not a priority.