I'm planning the same solution as Perrigrin. I'll have two "CC" buttons:
1) "everyone CC their current target" (for use just after teh pull as Perrigrin suggests)
2) "someone CC my current target" (for re-sheeping. round-robin among the mages) (obviously need to take-care not to break sheep...e.g. target only far-away targets, face my only melee char the other way, stop ranged DPS, etc.)

What happens if you cast polymorph on a target this is already sheeped? Does it extend the duration, do nothing, unsheep them? (And do banish/shackle/sleep/etc work the same way?)