I suppose I'll have to wait because I've tried breaking him out. I give the prison guard 100 penings then go in to talk to Rathbarth Ragnarsson, he tells me he's not going anywhere until I have the keys to the chains, I talk to the prison guard, ask him for the keys, and he yells "prison break". I then kill every single hostile unit in the city and I can't leave the city or enter the dungeon.
Boohoo.