I think that's a great idea, looks very usable. You wouldn't actually need a finger-nut at the pivot end of each rail, just tightening the screw that rides in the slot would hold it steady.
I can see the usefulness of the adjustable fence to allow you to put your mortice in any position... if you incorporated a couple of screws you could get really precise adjustments. However I agree with John, a set of shims of different thicknesses would be simpler. A set of 1/8", 1/4", 1/2" and 1" shims would allow you to shift the mortice off centre by up to 15/16" in increments of 1/16" - how often do you need to position a mortice more precisely than that? I guess you'd need to screw the shims to the face of the fence to stop them slipping out as you move the jig.