For my own writing I have a magic system where magic element is determined by personality and repeatedly reference a guy with Dissociative Identity Disorder that skipped between powers depending on his personality. That could be fun with a Dresden CHaracter too.
Spend 4 refresh on modular powers and say that at the beginning of the scene the player has to choose what type of channeling/ritual they know at a given time(and preset what each personality has.) All the RP possibilities too.
I'd be more than a little tempted to rule that this doesn't require modular powers (as a house rule). Instead, just buy "Channeling(GM's choice)", and define your high concept as appropriate. When this becomes a liability, the character gets the benefit of a compel -- or could buy off the compel to force his "round" personality into the "square" hole...
Note: most of the time, having the GM pick the element shouldn't count as a compel, because when it comes down to it, attacking with a force-based spirit attack instead of a fire attack has little game impact. It's largely when you, for example, fight that water elementals who are immune to water attacks where it becomes a liability.
Note 2: It would probably make sense to disallow rotes for the character; this would also provide a counterweight to the slight advantage of being able to spend a Fate point to gain flexibility in element choice.