Update here, someone on Mastodon mentioned the Fomu device (tiny FPGA in a USB-port-sized footprint) - I have one in the WK5 USB adapter and it is visible to the OS, although I did update the bootloader installed on the Fomu to 2.0.4 using a USB hub plugged in over USB-C to the side of the Pocket Reform, before getting it working internally.
(that tiny, tiny gold coloured thing with a green LED is the Fomu)
Bus 001 Device 006: ID 1209:5bf0 Generic Fomu PVT running DFU Bootloader v2.0.4
[ 15.377360] usb 1-1.4: new full-speed USB device number 6 using xhci-hcd
[ 15.485109] usb 1-1.4: New USB device found, idVendor=1209, idProduct=5bf0, bcdDevice= 1.01
[ 15.485130] usb 1-1.4: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 15.485136] usb 1-1.4: Product: Fomu PVT running DFU Bootloader v2.0.4
[ 15.485142] usb 1-1.4: Manufacturer: Foosn