In the Valiants and Darts, their a/c systems used the same components as the systems in the larger models, but did not have an EPR valve in the compressor, but the switches to cycle the compressor instead. There would have been a switch on the under dash unit to detect icing and adjust the temperature. Going backwards, of sorts, would be what you're mentioning on the earlier units. It might be possible to adapt one of the early '70s spec driers (with their low pressure switch) to the earlier vehicle and substitute the low pressure switch for one that would work to cycle the compressor instead of just turning the system off when the charge was too low.
I supsect that there might be some EPR valves calibrated for the R134a pressure range now. I also suspect that a knowledgeable a/c tech might know how to tweak an existing EPR valve for the more correct pressures of R134a too, just as all of the suction throttling valves for GM systems have always been adjustable (even before they started putting the adjustment know where you could get to it).
To me, one main concern on the earlier systems would be their condensors with fewer fins/inch and fewer circuits in it. By observation, that might generally be the case compared to what was on the middle '60s cars with more dense fins/inch count condensers. Adding a higher capacity condenser would probably help the performance of any of those earlier systems, I suspect. Having a constant air flow across the condenser is a must in slow traffic situations too, so the cooling does not go away nor the engine temp rise too much. Making sure that underhood air did not "recycle" in tail wind situations in traffic, by adding some ducting as Chrysler's service bulletin for the '72 Chryslers detailed, would be a good idea too.
Considering the factory a/c units (of Chrysler and later GM and Ford) which used cycling compressors, there should be enough parts to do a factory-look conversion to the cycling compressor system, I suspect.
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