... Take an aspect (almost certainly your Trouble aspect) to represent the fact you have a fallen angel in your head, add Demonic Co-Pilot and then take a power or two to show the benefits of the coin. If you take enough power to go over your refresh then you become an NPC...
I think I'd run with something along those lines. Except I think I'd treat the coins as a mental extreme consequence. Those rules seem to reflect the sort shattering, life altering weight of carrying one of those things around. And it's certainly not something one can normally get rid of. Like an extreme consequence it would take something special to do. I think if I laid one of these things on a player I'd have to do some building up to it. It made a great story, but the way Harry ran into one... Well let's just say Harry's player isn't likely to lynch his GM while the rest of us can't say that with the same amount of certainty and leave it at that. :)