.. but I thought about SRAM buffered by a large capacitor while not connected. A friend did homebrew such to connect it to a 80C31 once and I was thinking too to add a nice 8051 based circuit to give a bootloader for the SRAM portion for the emulated eprom.
The idea would be to upload the contents to the mcu circuit which will place it in the SRAM which I'd map as XDATA space. Upon turnign a smnall switch the SRAM woudl be hookedup to the EPROM output so the old console could see it as a ROM part.
But I still have to think a little bit about it.

Atm I lack the knowledge for CPLD/FPGA design but it surely woudl be an option as well!