Start him out on a slow path. Why would the denarian give the character everything he has at his disposal, when a little would do. It doesn't even have to be powers. Harry used the coin mostly for information, talking to Lash and using her perfect memory. Maybe give the character a small power boost that fits his overall theme, and slowly push him down the denarian path once he gets more refresh. And you can compel the hell out of the coin aspect, which leads to fate points for picking up temporary powers or simply invoking the coin aspect for all kinds of things.
Though I can see a character picking up a coin only go one of two ways in the long run: either he gives in to the denarian and becomes an npc, or he gives it up and loses all of the powers attached to it. Which might grant him new powers from another source in return (SotC, Soulfire, etc).