Then again, volts and amps are the necessary values. When you measure an engines output, You could either apply a known load, or you could apply some load, and measure it by torque and speed. Volts and amps give you a known load.You see where I'm going with this? Just because I have volts and amps doesn't mean I have the data necessary to declare xxHP.
I’m kind of thinking your original idea has some merit. I’d rig up a test fixture and try it