I noticed when I was doing a Heroic Pit of Saron that the army of Horde that fights the undead armies while you engage and fight Tyrannus give you rep of the same level trash mobs do in the instance. The funny thing is that they will kill them without your help, and I believe they cannot die to the undead. So basically if you have some friends who need/want to max their rep out on the tabard factions they can just bug Tyrannus and just sit just inside the mouth of the cave to leech the rep. The Horde guys killing the undead give you rep and you don't have to lift a finger helping them. I believe the only thing you would have to do is dodge Rimefang's breaths.
I'll see if I can test this and get a 100% confirmation, and give screenshots of what I'm doing.
In order to bug Tyrranus, from how I've noticed at least, is to engage him when he drops off Rimefang but BEFORE HE FINISHES HIS SPEECH TO YOU. He should then jump back on Rimefang and just shoot breaths at you while your faction deals with the undead army giving you rep.