Not _quite_ so lucky, but further than some other posters have got 😉
With further investigation, I was suffering from at least two different issues.
One is the windows 10 X52Pro drivers. to get the X52Pro recognised as a HID device, I need to reinstall the drivers after every reboot, or if I inadvertantly use a different USB port.
The other, currently unsolved, issue is with EyeX / ISE / vJoy calibration.
In E:D I now have up/down headtracking working properly, but not left/right tracking.
vJoy monitor shows steady movement with my gaze on up/down bar, but the left/right bar remains at zero no matter where my gaze is.
It seems like either ISE is not passing the correct values to vJoy, or that the vJoy calibration is incorrectly interpretting the values that are being passed to it.
I have logged a case with Tobii support, and look forward to working with them to get the issue diagnosed and resolved.