How are you changing the RPM? I ask because some of the high end Flux Vector Control VFDs on the market will provide you with an output motor torque metering function. So if you have a test bench for your machines, just use the largest motor you would ever need for the tests, connected to a VFD with FVC (also called Field Oriented Control / FOC in some sectors). If you are using a VFD anyway to attain thedifferent speeds anyway, you already have most of what you need, so long as you don't go for the cheapest one you can find. The highest accuracy will come from using the "closed loop" FVC/FOC option, requiring a shaft encoder on the motor, but the level of accuracy you are looking for was not stated. There are several out there that can offer "Encoderless FVC" that might serve your purposes if the required accuracy of your torque values is in the neighborhood of +-1%.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington