The way to go about it is to start with some behavior of interest that clearly involves control; a behavior like catching, pointing or walking. Then guess at the variables that seem to be controlled and then test this using some version of the Test for the Controlled Variable.
The interesting behavior for the 2/3 power law was fast drawing of ellipses (not just any voluntary behavior, as you state in the presentation).
You mention empirical tests: to show that the 2/3 power law is not a side effect of control of position, one way is to follow a fast target along an ellipse. Humans do it very well up to about half a second per cycle, but the position control model does not. Another way is to create a slow non-power-law trajectory, say constant speed over elliptical trajectory, and the model will track it, maintainting constant speed, which is a non power law trajectory. So, position is not the controlled variable, and the power law is not a side effect of control of position [with standard gain, delay etc in the loop].
The power law in the humans tracking helicopter trajectories is probably a side effect of the helicopter trajectories already being power law. In other words, the humans slow down in the same place as the helicopters. Other possibility is the noise (as in the paper by Flash that you quote).