You must have been watching an old re-run of the Mel Gibbson classic, Madd Maxx.
It was pretty cool how they had the blower do the same thing, be activated by an electrical switch, which used what looked like an AC clutch to turn the blower.
i am not sure that bit of movie magic would work in the real world. The blower takes alot of power to turn, and engageing it at such high engine rpm's would surely dissenigrate the clutch. it might very well work at low RPM's ?? Also, I would think you would also have to have a air by-pass, something that would normally be open and allow the air to by-pass the blower, but would close once the blower has turning. That would be very simple to make, weather it would work would be up to you and how much you would want to invest in the system.
I am running a Eaton M112 blower on my 4.6L Ford DOHC Engine. I had it ported and polished, and removed the stock pulley for a more aggressive 2.75" pulley, which should make 14-16 psi. To battle belt slip, I also bought a pulley tensioner relocation system, that moved the tensioner up and just under the blower pulley, so the belt is wraped arround 88% of the pulley or 316° of the 360° available.
What is your thinking on only running the blower part time?? Fuel economy?? Drivability issuse???