As the title says- I want to keep this in SE's face- it can be done. It shouldn't be called a "technical issue"
Maybe eventually, Hrothgar & Viera will have access to more headpieces in the future, or SE will see noone seems to be asking, so like many things- will be swept under the rug & forgotten about, just like players asking for less NPC exclusive hairstyles.
One fine example: Matoya's Hat. It's a bonus item given for purchasing the Encyclopedia Eorzea-- Something that can be obtained only by money needs to be designed for all races in mind. Yes, it's optional, but that's no excuse as to why it's limited to other races.
If Au'Ra can somehow still wear helmets through their horns- Hrothgar & Viera can wear more hats & Helmets- Not every large hat has to have Viera ears sticking out of it- but I can see most working just like how hoods work on Viera- Most headpieces & hats should be visible, if not all.
On an RP standpoint- I can see Hrothgar melding holes in their helmets so their horns can come through-
Viera & Hrothgar can have more hats & helmets designed to wear & actually be seen.
Agree with me or not, my point stands- Hrothgar & Viera need to have more hats & helmets visible.