Draco would have to be present after all. That means he wouldn't be able to cast the spell since his job would be to be the reason Rabastan was where he was at that moment in time.

Outside help is pretty much something they'll need here.

"I do, though I'm not sure Strahan could do it. He said he could only transform non-living things—people are beyond him. But Asar-Suti can change people, and getting him to do it won't be hard. It's not the revenge he wants but it'll provide him with a sick and twisted sort of entertainment to hopefully get him to agree to it."

Another world's magic then.