Hello mate, again I'll apologise for the almost 1 year late reply. Steam doesn't notify me of discussion threads being made so I just ran into this now.

Just to answer this in case anybody still cares, I didn't allow respirators to be wearable with helmets (or work like eyecover because of this) due to the fact that they will render over the helmet, over the chinstrap and the top of helmets. It especially looked horrible on side view.

If and when RimWorld adopts headgear layers, then I'll work something out. Right now, I have to fight the game's code just to make something that should already exist happen.