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Water is even better as a coolant if you can avoid it freezing and prevent corrosion.
I think Toyota's use a proprietary coolant from the factory. It's not compatible with some other coolants so flush well before swapping in something else. If you don't, you can get a mud like sediment in the cooling system if the old and new coolants react.
R134a won't work very well as a coolant in an engine cooling system because:
It can only exist as a gas at the sort of temperatures found there i.e. it's beyond its critical temperature and pressure.
The pressures would be very high, higher than in a typical car A/C system, because that's related to (the much higher) temperature.
If you can't use the change of phase (liquid to gas) to absorb heat, water and EG and PEG are better at absorbing heat anyway.
Normally, only liquid coolant is used but VW (for one) investigated using a vapour phase system where water based coolant was allowed to evaporate. That meant less coolant could be used, so less weight and faster warm up, and a smaller heat exchanger (radiator). I gather it was tricky to arrange the evaporation rate and heat transfer inside the engine.
"Air cooled" engines are in fact substantially oil cooled, as can be internal parts like pistons. One advantage is that oil will work at temperatures that would cause a water based coolant to boil. Otherwise, water is still a better coolant.
Maybe look at concepts like: specific heat, latent heat of evaporation, critical temperature and boiling point, particularly as it relates to pressure, for a broader view.
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