To be clear, I was having Cugel wait in ambush and motion for the rest of the party to do the same. Effectively the same as Wilhelm's plan if the bugbears don't actually check the cell, and since they couldn't see or hear us and them opening the door would be rather obvious to us, we should have a pretty big advantage (especially if Si'Nat hits them with a sleep spell as soon as they open the door). Of course, if Si'Nat blows our cover by casting a spell at them now, we don't have much choice but to fight them (and lose what seems like a good ambush opportunity, since any of them that don't fall asleep will be on the alert while we come out of the hiding spot to engage them).