I will need to think about (and experiment with) this a bit more, but at the very least I think I would need to be able to attach (and be able to retrieve from any process) some driver-specific display device properties (the Wayland output name it corresponds to, the compositor scale and native mode etc).
Any suggestions about how to achieve the above, without manually digging into the registry from the driver? Would some kind of small API exported from win32u.so for driver use be an acceptable option, so I can reuse some of the registry code from there? For example, something along the lines of {set|get}_display_device_private_data(device) to store/retrieve some blob of data from the proper registry key?