Just to clarify my comment about a "IIci-based ROM upgrade" what I meant would be to use the IIci's ROM as a starting point (since there's no point keeping an "unclean" ROM if the whole point is to raise the memory ceiling of the machine) but find and patch the part of the ROM that does RAM discovery and creates/loads the PMMU mappings so that in addition to looking at the built-in memory controller's address pool it recognizes the presence of an expansion memory card in the Nubus SuperSlot space and maps its banks into the appropriate locations in logical address space when 32-bit addressing is enabled. I didn't mean literally using the IIci's ROM as-is, because lacking a new discovery routine to locate and appropriately map the high memory at boot time it doesn't really buy you that much.
(In the sense that you'd still need to come up with a "ram-enabler" driver to wedge in early in the boot process and correct the mappings already loaded into the PMMU by the IIci ROMs, sort of a "MODE32-lite". That may or may not be cleaner/easier than trying to find the memory mapping code in the MODE32 extension and patching *that* so it does your expansion card discovery. Either is going to involve fairly deep hacking of binaries you lack commented source code to.)
One advantage I could see in principle of replacing the original ROM with a custom one paired to the expansion card would be that you could have the new ROM directly do whatever initialization is necessary to bring the card up without bothering to go through the Nubus/PDS declaration-ROM dance. Just have it do a simple sniff to see if the card is there and then either continue with initialization or crash to a Sad Mac as appropriate. Since we're dreaming here at this point you might as well also hack the video driver. Viola, no more screen corruption problem.
An obvious addition to this magic card would be an Ethernet port, of course. ;^) With that there really wouldn't be any need to make the card "well-behaved" enough to need a passthrough/pairing option with another board.
(In the sense that you'd still need to come up with a "ram-enabler" driver to wedge in early in the boot process and correct the mappings already loaded into the PMMU by the IIci ROMs, sort of a "MODE32-lite". That may or may not be cleaner/easier than trying to find the memory mapping code in the MODE32 extension and patching *that* so it does your expansion card discovery. Either is going to involve fairly deep hacking of binaries you lack commented source code to.)
One advantage I could see in principle of replacing the original ROM with a custom one paired to the expansion card would be that you could have the new ROM directly do whatever initialization is necessary to bring the card up without bothering to go through the Nubus/PDS declaration-ROM dance. Just have it do a simple sniff to see if the card is there and then either continue with initialization or crash to a Sad Mac as appropriate. Since we're dreaming here at this point you might as well also hack the video driver. Viola, no more screen corruption problem.
An obvious addition to this magic card would be an Ethernet port, of course. ;^) With that there really wouldn't be any need to make the card "well-behaved" enough to need a passthrough/pairing option with another board.