My assumption was that video frame period is low jitter, hopefully controlled by a counter divided from system clock or some other clock whose frequency we could find a way to control. Then if this clock frequency is slowed down for a little while and then returned (on one camera), the sync error will change.
It would seem this is how the GoPro's work in stereo mode.
The pixel clock and frame synch signals connect to the slave GoPro via a sync cable.
There are references in the firmware strings to speeding-up or slowing down the phase-locked loop that creates the system clocks.
Time to investigate this on the Canons.
We only know of three clocks, hopefully the movie task is driven by one of them and the frequency is changeable.