Canon lens MF and follow focus

[SmartWombat]

Has anyone looked into a follow focus rig for the electronic focus lenses?
Once you twist the lens focus ring past infinity or minimum distance, the direct relationship between focus ring position and actual focus is lost. So to work the FF control has to have end stops on the action.
Not only that, but if the relationship between focus ring and MF position is not linear then you can't accurately focus from the follow focus knob at all.
Anyone actually tried it?

These are frames from Canon DSLR video, where AF hasn't got a hope.
So I need accurate MF.