Is freezing a permanent issue for the sealant? CO2 isn't any colder than O2 or air when allowed to come to ambient temperature. And inflators that give you finer control of how fast the CO2 flows lets you add it slowly to the tire so there is less cooling going on while Boyles Law is showing itself.
I guess this question sort of begs the question, do people ride tubeless tires in sub-freezing temps? I suspect they do.