Cerebral Paladin
First Post
Maybe a prolonged skill challenge, representing sneaking up, assessing the target, and then getting in close enough to deliver the killing blow (which is a normal attack roll with a big bonus). Mess anything up on the way in, and you alert the bad guys. Mess up the assessment, and your attack is just a crit, not a coup. Miss the attack roll, and you're going to just kill him the old fashioned way.
I've toyed with the idea of running an assassination by playing out a normal fight, but with the whole thing compressed in time. Rounds become milliseconds, I guess. Of course, you want to have some mechanism to allow the victim to shout and summon the guard or whatever, and I'm not clear on what that would be--maybe a time limit? You have three rounds to win the fight? (Or of course if you escape the room or whatever.) And of course, this allows assassination attempts against PCs as well--your goal is to survive long enough to get the rest of the party into the fight.
Of course, it depends on what the goal of an assassination mechanic would be. I can see three different goals from the player's perspective: (A) I want to kill the boss without having to fight through the boss's defenses (that's what my proposed system would handle); (B) I want to avoid losing hit points/resources by not actually fighting the boss; or (C) I want a different mechanic to represent the different sort of kill, to make it feel special and awesome.
For B and C, something more like a 'kill challenge or the system you proposed would be better, but I would worry that it would be either anticlimactic or create weird issues. "You approach the dragon's lair." "Great, time to try for another assassination!" "Y'know, it would be nice if you fought some battles that weren't failed assassination attempts..."