It seems pretty logical that since the system was never designed to have separate pauldrons, it treats the armor for the chest and arms as one unit. It would have to be a full reskin, just like adding a cloak to an outfit. And would add a full chest piece to the database. Sounds like a lot of work since there are a ton of armor sets with shoulders. If they were separate, it would be easy like with heads where it pretends the head slot doesn't exist.