I spoke to OWC tech support, their workaround is that if you can reinstall the original Apple-supplied drive and then install Monterey on it, then it will update the EFI firmware for the device and then you can install Monterey on your Big Sur volume. It seems this is also mentioned on
Apple support thread as a successful solution.
Unfortunately whatever mess that happened so far with the Monterey installer failure has deleted my recovery partition and made the network recovery unable to see my drives. It looks like I'm going to have do get some hardware to fix this, and of course Apple has stopped all genius bar appointments.