It's been a while since I used to hack auto a/c systems. My recollection is that a blend of propane and isobutane is required to get somewhat near the same pressure/temperature curve as R-12. Doesn't straight propane have runaway high side pressure at high temps? Depending on the percentage propane in the system you may be putting the system under quite a bit of stress.

Second, isn't grill gas quite wet? You're adding water to the system along with the propane. Water in combination with R-12 is not good, the two react to form acids that then corrode the system from the inside out.

FWIW I'm in the camp that is not the least bit concerned about flammability of hydrocarbons in the a/c system. Of course, I don't smoke either