I like it.

I had considered putting ribbons on the rudder/fin myself as a test, but have been deterred by the weather. For me, your simple demonstration passes the first order "smell test."

For my money, the important factors creating a force for left yaw seem to be (in no particular order):
* Spiral slipstream on asymmetric tail feathers (more rudder/fin above than below the thrust line)
* P-factor in high-alpha attitude (axis of prop rotation is above velocity vector)
* Uneven loading on landing gear caused by prop torque (takeoff, esp on grass)

There may be second, third order effects, but that's probably splitting hairs. These might include ground effects on the slip stream as it affects the tail feathers and the P-factor.

Once airborne and in level flight only the first factor would apply. Forces from gyroscopic precession would come into play when there is movement about the yaw or pitch axis. These should be easy to calculate (for some) based on prop mass, prop rpm, and speed of yaw or pitch. Their effects, however, are almost certainly masked (to the pilot) by the control inputs otherwise needed to manage the airplane through the maneuver.