On the AppleTV run Zwift, with bluetooth, chose a flat route. Do virtual gearing in zwift. Run SYSTM on the windows laptop with an ANT USB dongle (from Garmin btw). Then pair via ANT with “Power” and not controllable. Now zwift controls the resistanse, but SYSTM will “follow”. Use virtual gearing.
I have purchased a Zwift cog and tried my theory in practice. I would like to confirm my hack. It is possible to do it like that. I also discovered that you don’t have to start a real ride (on a flat course) as long as it is paired with Zwift. However, you only get a visual indication of the gear if you actually start a ride. You can feel the gear changes even without the visual. Next time I do Full Frontal, I will do this.