The tabard only dropped from the mob of Scarlet Trainees that would appear after slaying Herod. The dungeon was updated with Mists of Pandaria - which means that Herod was removed as a boss and the fight mechanics changed. There are now no Scarlet Trainees that appear after the boss fight concludes.
I contacted a Gamemaster, and the GM was able to confirm that the item is completely removed - not assigned to another mob's loot table - from the game.
It seems Blizzard added the tabard to Harlan's loot table in Patch 5.3. Rejoice!
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