As a dracthyr, I tried to use the Ebon Blade tabard transmog but found that it wasn’t available despite having the appearance owned.
I can still transmog the tabard on any other race and class, and can access the reputation/faction as well.
I tried to do the Shadow Vault quests in Icecrown to see if I had to raise reputation for it, only to find I could never even access the faction reputation for Knights of the Ebon Blade at all - but I could get Argent Crusade reputation, and therefore can use their tabard. The Knights of the Ebon Blade faction doesn’t show up at all under the dracthyr’s reputation page for Wrath of the Lich King.
This has been unavailable since their release on November 15.