Adding that water and propylene glycol have been used for this in a few places.

True, that’s pretty common in industrial and commercial settings as well. Though here I’m thinking more in terms of a residential setting. Relying on a secondary fluid loop would likely raise the cost quite a bit due to the extra components required.

They’ve raised the global warming issue about current refrigerants but what really matters is the economics of isocaloric vs. vapor compression because there are more modern refrigerants that have quite low global warming potential.