I tried three different computers, two different adapters, one rs232 port that was integral to the motherboard, and three different leads. Saint Ed of Meridian kindly and patiently talked me through all the various permutations that were possible, even, amongst other things, suggesting loading the core mfu file on its own. The G98DH, helpfully, would light up cheerfully to announce "Erasing..." followed by "Downloading..." and that would be it. Fupload on all three computers would spit back "!R connect failure" or "10000ms non-response time-out". It proved impossible to revert it to its functioning, pre-upgrade state.
It is entirely possible that there was something wrong with the unit previously - I have already mentioned my HDMI issues on this forum - but when the booming voice from the Oracle above intones "It sounds like your bios is corrupted," my inclination is to say "Yes, your Worship".

Main System: Trinnov Altitude 32-8-16, 271, 2 x 8KXE, 1 x 8KPP, 6 x M6 Surrounds, Rel No.25, Oppo 203, Zappiti Pro 4K HDR, Roon, WiiM Mini.