So this similar workaround works for me using Zwift Play controllers and a Kickr.
No need for Ant+ dongle.
Because Kickr can support multiple BTLE connections you can give trainer control to Zwift on a flat route (Tempus Fugit), and to completely eliminate any gradient you can also turn trainer difficulty to off. This would allow you to use this workaround on any route in Zwift.
Simultaneously run SYSTM via computer (I use a MacBook) but toggle trainer control and elevation off on the settings screen.
I opened Full Frontal and tested this. I am able to use virtual gearing from the Zwift Play, and by varying my cadence and virtual gearing accordingly, I could then complete all 4 tests from Full Frontal with valid results.
If you don’t have AppleTV AND a computer, I imagine it would work the same running Zwift from a mobile device and SYSTM from the computer.