Thinking about the models of 4e, I would assert something a little odd. By choosing to become an NPC, you quite literally become a "monster" for purposes of combat. Arrange with your GM an opportunity for you to become, thematically, an elite or solo (depending on what level of assistance you want) and to translate your character into a monster.
The reason why I recommend this is that any given party will absolutely gank any one of its members that turns on them in an even fight.
Consider the following:
A striker at level 14 is expected to do (8*14+24)/4 34 damage with their at wills after accuracy is accounted for. As you are the controller of your party, there are inevitably one or two other strikers who will, as they say, "faceroll" you on their first action. And it would be absurdly difficult to conceive of two strikers (not to mention the rest of them) dropping action points and two dailies not being able to drop a controller with no defender support.
This, honestly, isn't a very interesting outcome.
The other case is that you engage them outside of a traditional encounter, using surprise and poison to play "Ten Little Indians." (Interesting book by Agatha Christie). This, while interesting in a role-playing setup, features no combat per se. Quest XP would be an appropriate reward if they managed to drop you.
The middle case is that you, through powers of system-magic, turn into a mob. In the most interesting case, you're an elite with a chamber of specially prepared traps and hazards. (And minions. Can't forget a layer of tasty tasty ablative minions.) This is a nice, if counterintuitive, case because it provides for a satisfying combat for both sides. As such, your XP should be commensurate with the monster that you become.
One happy upshot of this is that whatever evil deal you struck can actually be shown to the players as you undergoing a magical-girl (tm) transformation sequence, explaining why your loot evaporates. It also gives you a final moment of glory before the rest of the party cuts you down like the monster you now are. :)