Having decided that the direct-connected mega2560 will be too slow, I will change tack and utilise the above dual-port SRAM module. The Arduino Mega2560 will contain the .hex file within it's flash memory and at power-on will transfer this to the DP SRAM. There will be a flatribbon cable+dip28 socket to connect to the 80C552 board - or any similar embedded cpu that uses this is similar eeprom.

The SRAM will be treated as ROM by the device being 'revived' - with some creative coding it could be possible to modify code on-the-fly via a serial console back into the Arduino Mega2560.