Though, I'm not a hardware engineer at all...
 
> As long as we're talking pipe dreams...
>
>        What about a 'helper' ROM on the daughter card that does any CPU
> specific setup so that the motherboard ROMS work as-is... The daughter
> card could also have local SDRAM.

I looked at 3MIN/MAXINE CPU module closely.  It has 25MHz 3000A/3010A
made by Performance Semicon (Where dit they go?) One LSI Logic custom
chip L1A6406 (DC7201C) seems designed and ordered by Digital, probably
is called CPU ASIC which is hard to be replicated.  On another side
there is a small AMD chip maybe programmed logic.  ALL other major
chips seem Moto's SRAM for caches. IOASIC and TC ASIC sit on mother
board.  Intel N27C20 and S(ignetics?)27C751 are socketed parts,
possibly reprogrammable and contain REX TC ROM monitor.

>        I guess the hardest part would be re-creating the
> MIPS-BUS-->TC/MEMORY ASIC. Might be possible to use the old ASIC with a
> newer processor as long as the CPU BUS was the same (or very similar) and
> could be slowwed during ASIC accesses...

Tohru Nishimura