Okay I might be wrong here so if someone could check over this and correct me, here's my thinking for it:
Power is the torque at a given speed, therefore to calculate the power output you need to measure the speed and torque.
If you was to connect a DC brushed motor to the output and were to put a MOSFET and power resistor across the terminals you would get a variable load. (Using PWM.)
You can measure the voltage drop across the resistor and mosfet to get the current flowing through in and hence work out the power dissipated from the resistance of the resistor and the resistance of the MOSFET when on.
If you know the efficiency of the motor (Does this change over the motor's life ?) at the current RPM, you can work out the mechanical power used. (I'll assume a motor has the same efficiency generating electricity as it does using electricity.)
From the mechanical power and the speed, you can work out the torque at the given speed.
Are these assumptions correct ? I'd also like to make a Dyno for stepper motors ...
I think that what you would do is open the throttle fully and use the adjustable load to get readings across the full speed range. I don't know how you would do this with a stepper motor as there is no "full throttle", but instead you must tell the motor what speed to go at ...