Use the Escort mission detail, then set Betray to 'True'.
Use escort, and have the escort betray on arrival :-)
Cool, I'll give that a shot. 
Hmm, ok, not quite working. First, he doesn't attack the players right away, which is what I was hoping for.
Second, the escort does work with the big orange box type exit, which is what I have. Can't really change the map either, this is one of the few maps that work for me (the only one for this mish, really).
Other ideas? Can the escort be made to attack the players right away?
I don't see any way to have a captive fight the team, along with the group that captured him. Like the Cim missions, where you free the captive but then he turns on you also.
Any ideas? I'd really like to use the Rescue objective if I could, that placement looks better. Fight a Boss looks kinda fun if I set it up like that.