If only life were that simple. Stepper motors do not behave like most other motors.
Other than exceptional cases of high speed operation at high voltage, stepper motors
will act as nearly constant power devices. This is why it is desirable to invoke reduced
current at idle. Even if you could splice the meter that you were looking at into the DC
power system inside the PMDX-340, it would not tell you much.
The only way to get an idea of the torque load on a stepper motor is to look at the
phase relationship between the drive current and the back EMF, a task that is quite
complex. A few advanced driver chips can do it, but the readings still need to be
massaged to take into account the inductance and resistance of the motor windings.
With brushed DC servo motors it is much easier to measure their power consumption
and all you have to do is correct for the resistive losses.
It is also fairly simple to measure the load on an AC induction motor like those used
as the spindle drive on many machines.