Tried CO2 (the aluminum tank from the MIG, with a different regulator (also works for adding fizz to drinks and fruit).. started running the little MIG with straight CO2 because the CO2 was free and just never changed that one back to mix), here's what I found (warning opinion follows/your mileage may vary);
..it takes up too much space (more than 4 times that of a small compressor).
..airing up is, mostly, only done once per trip so kicking back in camp with a brew is a satisfying way to kill that time, needs a latching/screw on tire chuck (before leaving the next day).
..air pressure is not typically needed for anything but, activating the air lockers and airing up tires; very rarely for reseating other people's tires on their rims (I carry hand tools, more compact than air tools, for emergency repairs).
..while CO2 could power the air locker ...why? when the locker compressor is so much smaller (and mid size and bigger ARB compressors have so far been utterly reliable (have had 3, the newest has not yet been installed), over near 30 years)... drain the ARB compressor tank, all air compressor tankes, really, as required, to prevent water buildup.
..on a previous vehicle I did the air conditioner compressor conversion/on board air thing; it was much better at most things than a small electric compressor and still freed up significant space, not a big enough advantage for me to go through the trouble of adding a second, converted, air conditioning compressor just for on board air though...
Enjoy!