Can't say I have, but those pieces of software sound like they might be worth a shot.

Found the "Spectra 333" Upgrade CD - I'll let you know if that helped.
Alternatively I'll attempt to get a keyboard controller that I know works with MMX systems (namely Holtek HT6542B or VIA VT82C42N)

EDIT: Sounded to good to be true ... the Evergreen Spectra lists my system as incompatible 🙁

^^ Oh well, nice try. I think there was a similar kit from Powerleap, but I don't know if that was more compatible. That might be worth a look if all else fails.

Oh well, another conclusion ... seems like the !A20M signal gets generated by the chipset and not used from the keyboard controller (the A20 gate from there is actually unused). Might be worth a shot to bypass the chipset and go back to the keyboard controller - but that doesn't appear too promising tbh.

EDIT: I'm giving up for now. Rerouting the "A20 gate" from the keyboard controller to !A20M led to the machine not starting anymore. At least by now I figured out that my Atmel 29C010 flashroms work on the machine (tried some SST 29EE010 before). I've also tried the BIOS from an Acer V50LA (same chipset, same general specs) which gives me a single beep but nothing else (even the keyboard LEDs remain unresponsive).
Oh well. Maybe I'll get a PODPMT66X166 or PODPMT66X200 for cheap at some point which I'd consider worth a last try.

Short update: The board doesn't work with Pentium MMX Overdrive CPUs either.