Viera and Hrothgar are normally able to wear this style of headgear, but this model is not visible.

Date & Time: any
Frequency: always
World name: any
Character name: any (issue is with character model)
Retainer name: -
NPC name: -
Monster name: -
Class/Level: -
Party or solo: -
In-game time: -
Area and coordinates: -
Housing: -
Steps: n/a

This affects six gear pieces altogether:
Alliance Visor of Maiming
Alliance Visor of Striking
Alliance Visor of Aiming
High Durium Visor of Maiming
High Durium Visor of Striking
High Durium Visor of Aiming

While Viera and Hrothgar are of course notorious for not being able to equip all headgear, the deciding rule seems to be that hats which cover the top of the head cannot be visibly equipped, but masks, circlets and other pieces that do not affect the hair can be equipped.

By this general rule, the Alliance visors seem like they should be visible, but they are not.