In World of Warcraft: The War Within, Blizzard announced that players will be able to learn transmog of any gear type, even if you cannot equip it — unlock plate transmog on your mage!
Currently, players who wish to unlock transmog must acquire the transmog on a class that can equip it. In a press release, Blizzard has announced that a new change to transmog will let players learn the appearance of any gear type, even if they cannot equip it.
This means that it should make it much easier to unlock all the transmogs in the game, especially on class specific items like old tier sets.