As you know HPA tanks are rated for either 3000, 4500, or 5000 PSI. CO2, regardless of how full it is (for the most part- overfilling is a concern), is always the same pressure (as long as the temperature is the same). At room temperature, it's about 850 psi. At 120ºF, with a CO2 tank that is 100% full (which you should never, ever do, 75%-80% is a proper fill), the pressure is still below 2k.
So will it work? Yes, the tank will hold it. However, if your regulator is not up to par (and most aren't designed for holding CO2, which can cause seals to break if liquid touches them and expands), then you can have a bad situation.
All in all, it is possible, sounds like it's been done often enough, but I wouldn't do it unless I knew my reg VERY well
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