That is really cool. Is it possible to explain in simple terms what you changed in the ROM to make it work?
I'm kind of surprised that the lack of 12 X 12 support was fixable in firmware. Why in the world didn't Apple put that in to begin with?
Yes, the Q840AV is different. It seems to have a memory control system similar to the later PM7100 and PM8100. At least, it behaves similarly, treating 64MB and 128MB SIMMs as 1/4 of their capacity. Apparently, it only has support for 11 X 11 memory banks.
Hmmm. The PM6100 has support for 64MB and 128MB SIMMs and used the same ROM and chipset as the 7100 and 8100. However, on the 7100 and 8100 the upper address lines run through some kind of CPLD before going to the ROM slots. So, I do not think that the memory addressing in the 7100 and 8100 can be changed with firmware, but who knows.
But, if the 840AV does not use that CPLD arrangement, then perhaps all that's needs is the same tweak that changed the memory addressing in the Q610 to 12 X 12.