OK, I finally have some good news. I've seen the exact same hardware work with #Signet under Debian 12 and fail under Qubes 4.2. This confirms it's a software issue, not a BIOS/motherboard problem.

This is good because it means I should be able to figure out the root cause and ideally work around it with some updates to the code that I control.

If not, I should be able to at least describe what one has to change in order to be compatible with Signet.