Tried algernon’s suggestion from this post. - that is, holding down PROG, plugging in, and immediately flashing FW (via CLI). It did not work.
However, on my first attempt, I saw this error:
Looking for USB device with vid=1209 and pid=2301
Examining usb-FTDI_TTL232R-3V3_FT9FQWWB-if00-port0
ID_VENDOR_ID 0403 != 1209
Examining usb-Prolific_Technology_Inc._USB-Serial_Controller_D-if00-port0
ID_VENDOR_ID 067b != 1209
ERROR: I couldn’t find a USB device matching the keyboard’s USB Vendor and Device IDs
I couldn’t autodetect the keyboard’s serial port.
On subsequent attempts, I did not see this error. I just saw the normal output instructing me to hold down ‘Prog’ and release it (and when I did this of course nothing happened).