My thoughts so far:
Get the Beast Change power so you can switch between two different skill sets.
Take Human Guise - it's free anyway, and covers any contingencies.
Invent clever Aspects, keep a decent Refresh rate, and refrain from purchasing Demonic Co-Pilot, so you can Invoke those Aspects for bonuses rather than rely on the baggage which Demonic Co-Pilot brings. Failing that, convince the GM to allow a "nicer" Demonic Co-Pilot for maybe 2 Refresh.
Sam is on the night shift at his job as a security guard. He doesn't know it, but a RCV is sneaking up behind him. Billy (the spirit) alerts him and the fight ensues on even ground.
Could be the player Declaring/Invoking his "Billy the Spirit" Aspect to make the Alertness check.
Sam is in the midst of a shootout with a few thugs when Billly enhances his host's vision and accuracy so that Sam can take them out easily.
Could be the player Declaring/Invoking his "Billy the Spirit" Aspect to make the combat check (if Billy is just another pair of eyes) and/or Beast Change to switch to a new set of skills (if Billy is a combat monster).
Billy is in control and taking out a some other thugs, killing them all, when Sam stops him. Sam appareantly wants to show mercy. Billy tries to resist, but Sam takes over again and uses the situation to intimidate information out of the other thugs, then calls the cops.
Situations when your spirit is in control and the host body wants to stop it sound like a GM or Self-Compel which is bought off by a Fate Point.
Buy any Magical Powers without any restrictions, as it seems to me that there's not really going to be any limit on when they would be used. And situations when those powers would "go away" are probably going to be either Extreme Consequences, or a Major Milestone during which you shuffle your powers around.
Edit: Admittedly, this is the less fun, "no need for a custom power" method.
And since we're using Bob as an example, the player could be getting lots of Fate Points from accepting Compels from his "Billy the Spirit" Aspect or any Aspects which reflect the trials of having Billy along for the ride.
While it is tempting to build this from the perspective of the player being the Spirit, who relies on an NPC host to do things, it's just going to be easier to do things from the Host POV.
Now, if you want to be able to Switch bodies and return... well, then we're talking some serious Powers.
Also, this is just a suggested way to do it in a way I think may be an elegant solution to what I feel is more of a role-playing challenge than a power-build challenge. Now that I've laid this on the table, if a new Template or Power is going to suit more, I'm happy to help develop it.