It's been a long time, but weren't the Zul'gurub sets from quests like AQ40? As far as I know the AQ40 ones are repeatable and I don't know if the game will be able to check if you've ever completed them.
But there was also a crapton of gear that I obtained while soloing Zul Gurub that I ended up vendoring due to bag space. I really hope they add new sources for unobtainable item models.
I don't have high hopes that Blizzard will hide unobtainable transmogs in the new system. The pet and mount tabs shows still stuff from very old promotions, Asian-exclusive promotions and other unobtainable pets/mounts.