What do (can) you change to make headgear show up on your character if you change your PC to say someone like Uthar/Dustil/etc..... where any headgear doesn't show up? I've noticed that some of the unique models, for instance Gadon, will show as wearing a mask during gameplay, but others will not (including the more generic ones used for speaking characters throughout the game---like child_male/female, Commoner1_Mal_Asian, etc...). thanks.

I don't think that they have the headgear hook in thier models so what you would need to do is put that in there. I could be mistaken though. Hope this helps.;)

How exactly do you make the required change?

unfortunately you can't add mask hooks to heads.