I figured out how to free Saul. You need to have visited High Hall and fiddled with the control panel on the teleporter there, even though it does nothing. After that you can change the teleporter in the prison camp.
Larian didn't anticipate one possibility, though: I accepted the quest to kill Saul, I changed the teleporter to point to High Hall, told Saul to go, then quickly ran up and changed the teleporter back to the Slayer camp before he reached it. I was hoping he'd go through and get killed by the slayers, but nope, he went to High Hall anyway!
