It's not about equiping the item. I can farm it on a shaman and equip it on him. In my understanding that will unlock the image and then allow the hunter to use it.
It is like 1 shaman set has almost all but 2 pieces available to hunters just because Blizz didn't add more items as they weren't needed on the loots tables it was not to prevent transmogs to that set.
In the Wardrobe thing they are adding you will be able to see all the items and what will happen to say the legs piece that you have unlocked with the shaman tier but is also available as an offset piece.
Will the game say you didn't unlock it with an off-piece so you can't use it ? I highly doubt it.
It will be even more confusing for the 4 leather wearers out there this peace is ok because it's from a tier set but has an off-set piece and the other piece is not ok.