My guild has Sarth 3 drakes (10 and 25) on farm now, I do think it'll be doable as a multiboxer but only as a DPSer -- no way you could control tanking, DPS, and healing it. The hardest parts of the fight are the tank/healers jobs, DPS just has to move out of void zones (strafe ftw), avoid lava walls (follow ftw), and DPS the right targets, and take portals at the right times (drake 3).

imo healing multibox is actually one of the hardest, unless you control 2-3 guys who are just spamming the MT, because target selection is a huge part of healing. Just spamming CoH/PoH/chain-heal-on-self is not a great healing strat and fails in many encounters. When I play my resto shaman I usually am #1 on heal meters and I use Grid/Clique, and probably make 10-15 "heal decisions" a minute, don't really see how you could effectively do that as a multiboxer. Of course boxing a healer who spams heals on the MT works great while your other guys DPS.