It doesn't even have to be tied into their own background.
Just using that as an example. I mean if one PC decides "I'm going to go work for the Fae/Mob" and turns to a GM and goes "If you want to force my hand due to my connections, go ahead".
We're basing it off Fate 3.0, but there will likely be differences from the final Dresden Files game as published.
By different from the game as published, do you mean Rules wise, or just Universe wise (since I know it was said previously that you have your own ideas about Baltimore's supernatural makeup and the like)? I can also imagine that Fate would likely have to be tinkered with to fit the MUSH programming.