might as well stop the whole thing from turning if it's not boosting. Mercedes did this
That's a useful mod.
If the blower stops, obviously almost all the power lost in the drive comes back (except for belt friction to the clutch, and the clutch hub bearing).
The bypass, of course, now sees only vacuum (since nothing is satisfying engine demand under the rotors), and remains open, diverting air or mixture from the carb right to the manifold no matter what the throttle position.
IIRC Magnuson sells a stand-alone bypass valve.
The Mercedes M62 application (2.3 liter L4?) shows that the bypass area is enough to cruise the car with the blower stopped.
I'll bet the Mercedes clutch is keyed to vacuum so that the blower engages on full throttle, and the boost closes the bypass.
If you're going to use an A/C clutch (which is solenoid operated by a switch) I'd use a relay and a kick-down switch on the carb secondary linkage etc. rather than a vacuum switch, which will operate differently depending on which gear, hills, etc., and may induce knocking.
