That there would be some issues. First, the connection to the tank.
You would have to adapt that side to the CO2 tank. I am also fairly sure that a compressed air regulator isn't built to handle the pressure of a CO2 tank.
Next, would be granularity. If you look around the boards, guys are running their rigs anywhere from 5psi to 15psi for dispensing of beer. If the air regulator doesn't do fine grain adjustment at those low pressures, you will be humped.
In short, I wouldn't do it. Not worth the hassle or uncertainty.
Micromatic sells a line of inexpensive regulators that can be had for thirty seven bucks plus shipping:
Or you could get their premium model for fourty four bucks plus shipping:
Doesn't seem to me that saving twenty bucks is worth the time or effort, especially considering the work you put into making the beer.