UPDATE: I tested the system yesterday with the organisers of the conference. This time I had 2 external monitors (one to pin the zoom participant, one as a projector for my studio) and it didn't work. : (
Screen Capture was constantly getting confused about which screen to capture. Even when running in a background scene, sending video over a Broadcaster, when I changed the Listener scenes, the output from the Screen Capture would change to a different screen.
So now I am running Zoom on 2 machines. One has a HDMI output to a capture device on the other one. I pin the Zoom participant to the capture device and then the video shows up as a Video In source in Isadora. I then use Syphon Virtual Webcam to get the output from Isadora into Zoom.
It seems to work OK now I'm not using Screen Capture. Time for more remote testing.
EXTRA UPDATE: as long as the host doesn't Spotlight anyone, then the pinned video feed works fine!