Gee, any decent electronics guy has a bench top variable DC power supply. While mine is only 2.5 Amp capacity , it ranges from 0 to 30 VDC , plenty enough to test the voltage section of an over voltage unit. If this unit does look at current, that is easy to solve too

I'm taking issue with your statement " There is no valid test for an OVP. " not that trying a known good unit isn't faster.

If one can mimic that goes on when the OVP is in the car, one can test it out of car. This isn't like we are trying to test a house numbered IC with an unknown pin out in a ECU.