A motion sensor on the driven part of the shaft would work as well. The instrumentation to implement a motion sensor to lock out the motor is beyond my experience, however motion sensors on screw conveyors were commonplace at the Wastewater plants I used to run. The Interlock system would prevent a downstream conveyor from starting if the Upstream conveyors weren't running, however in the case of a broken shear pin, only the motion sensor will notice the mismatch and shut down the affected feed screws.
If your customer is routinely snapping shafts, there is a problem. If he is snapping shear pins, you may be able to find one with a higher shear strength, though this is analogous to putting in a larger breaker in an electrical panel.
A power meter may not be able to tell the difference between a shaft running out of water an empty tank situation versus a broken shaft (ie no load). I'm also guessing that an audible alarm or something tied into a computer would be desirable to detect a broken shaft condition.