OK, finally some progress! Using the test machine, which is still set up with #Qubes 4.2 but without a USB qube: I made a Fedora-based qube, passed through one of the USB controllers, and it shows the Signet as "[unknown] [unknown]". It doesn't use the udev rule (or it does but the device doesn't match).

This is using the exact same kernel as my previous test with a Debian qube, where the Signet worked just fine.

So it seems to be related to Fedora, and unrelated to the kernel version/configuration.