I discovered this accidentally a few minutes ago. And it's rather simple.
1. Have someone with the mog you want.
2. Run a dungeon with them. Get your item to drop. Has to be rare or epic and the mog must be equippable by both people.
3. Have them take the drop, equip it, and transmogrify it.
4. Have them trade it back without unequipping it. In the trade window it WILL NOT show the transmog and will say 0/0 durability. You keep the look! If you're across servers, just leave the dungeon and queue up again or port back in.
I have not tested, but it may be possible to have a level 85 run you through and transmogrify the item into one of his items prior to trading back, as long as the item has no level requirement.
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