Last letter from the producer confirmed that.

As long as the base gear and the gear being copied occupy the same number of gear slots, you can copy the appearance of any gear equippable by your current class or job.
I camped the shroud for weeks to get enough mantels for the complete armor
and now that?! Nobody can tell me, it's a technical limitation as the armor itself
is a chestpiece and could just overwrite hand legs and feet in their appearance.

Well, good to know that was all for nothing.