For Saga I simply ruled the PCs were helpless, meaning they could be coup de grace'd.
For this I really like the idea of a Coercion check, but this brings up the controversy of forcing the PCs to surrender via a skill check...
I don't think I'd ever force the PCs to surrender via a skill check, but I do like the idea of giving them fear setbacks based on failures to a Coercion roll.
That seems like a great way to run it really, as well as the other way around (i.e. you've got a Nemesis you don't want surrendering on a simple skill check either). If it causes penalties, players will still feel the skill was useful.
PC surrender and similar subjects all revolve around one basic roleplaying problem, you are taking de facto control of a player's character away from them. Since that's ALL they have in the game, a player is going to be rightly nervous about the possibility of the GM taking it away. If it's the equivalent of "lose a turn" in a standard game (i.e. something brief that won't perma-screw the player), most folks can live with it, but you need to make it clear they will get some control back soon-ish or they're gonna see themselves just sitting at the table doing nothing while the GM runs a railroad.
The "perma-screw" thing is important also; one reason surrender is unpopular is PCs are usually facing villains, and surrendering to somebody who will cheerfully torture/murder you anyway is quite probably stupider than fighting against horrible odds, so guess which option PCs take more often? You have to make it clear surrender is a possibility for survival/another chance later, or at least make sure PCs do get a chance to resist harm (i.e. escape from their cells before being fed to a rancor, etc.). If they roll their own dice, a good player can accept the consequences of failure because it's their failure; hurt them without them having any chance to stop it and they will never trust you again. But always treat surrender carefully, loss of control of a character always tap dances on the heart of the GM/player "contract", and it's easy to screw up and railroad. Communication is key!