If you have a better idea, I'm open to it. Yes, you can sample slowly if you can tolerate a longer-term average (like you would need to display on an LCD). I realized that even with low sample rates, I would sample every part of the waveform eventually. But I needed an "electronic circuit breaker" that had to respond on a cycle-by-cycle basis.

I proved that I could respond in one cycle by connecting a bridge rectifier + resistor + FET across the load and connecting the gate of the FET to a pin on a PIC development board then bringing the the gate high for 17 ms. It worked and the circuits are in the hands of a happy (and very demanding) military customer.