Here's a somewhat different idea I just had, to try to approximate the information I'm looking for, possibly on the cheap...

We know the torque curve for a brushless motor with static timing is approximately flat, starting high at the stall torque, and going down to zero at the free-running RPM, right?

Well, figuring out the free-running RPM isn't difficult, with the max RPM feature in most ESC: just remove the pinion, rev it up briefly, and you can find out.

If we assume the torque curve is flat, all we need is another point, preferably at the low end of the RPM range, and we can get the (very) approximate torque curve!

Now, the question becomes this: what's the simplest way to find out the stall torque, or the torque at some low RPM?

Since our other torque data point is zero, we don't even need a real unit, only something that we can compare other motors with...

Any ideas?