I have a dynojet and can run a negative dyno test to get drivetrain losses in inertia mode., I am not sure how the mustang derives those numbers using a load cell.
I work on other types of machinery that uses the same strain gauges that a lot of load cell dyno's use, they need to be calibrated between 2 or more data points and on a regular basis.. I know some
other dyno's just have a zero'ing function but that doesn't mean anything. I have the load cell option on my dynojet, you zero it, and then put a fixed weight on a torque bar to see if the gauge is reading right.