It would probably require something like AU Ner'zhul's mate Rulkan, currently a shaman, to turn to the void like he did for a Shadowmoon representative... but even then, she's currently allied with the Alliance so it doesn't help the Horde case unless she also switched allegiances as well.
But in terms of other Mag'har clans, one possibility may be Jorin Deadeye, son of late Kilrogg Deadeye, who currently leads the MU Bleeding Hollow clan as its Warchief. His cloth equipment seems to suggest he's a priest, acolyte, or necromancer. He currently resides in Garadar, Nagrand and is friendly to Horde players.