Amartya Sen has written extensively about how to do just this, though he wouldn't call it utility either (it's one of the cornerstones of the capability approach). He formalizes it in terms of the real option sets available to an individual rather than "free will" though. The main difficulty is how to quantify and value different option sets. (You can't just look at the size of the sets, because different options are likely to be differentially valuable qua options, and you need to incorporate that somehow.)