Definitely not a good idea injecting HPA into a mag filled with propane. Air contains 20% oxygen which still isn't a heck of a lot of O2 at 120psi, but it would be about 8x more than you could get in with atmospheric pressure air.
Setting reg pressure to much higher than 200psi could start to become quite dangerous. Our mags do not have anything close to the engineering and quality control of paintball bottles. Unreg'd CO2 adaptors present a significant hazard if they are used to push liquid CO2 into a GBB mag.
Pushing in reg'd 120psi CO2 would not give much of a constant pressure boost after the first shot or two. It would be vented in the first shot or two and you'd be back to the same propane evaporation system (CO2 is not soluble in propane).
Want nearly free GBB gas?