Adding pseudo-ISP into old 8031 boards...

[technix]

I have a second-hand oscilloscope that is powered by a 8031 and some external mask ROM. And I found a bug in it. Do you think this being a good idea to fix it:

1) Cut the trace to the EA pin on the 8031. Maybe the trace to WR pin on the ROM socket
2) Copy the contents of the mask ROM into some kind of EEPROM, Flash memory or FRAM that does not require VPP, for example FM16W08.
3) Replace the mask ROM with the storage chip above.
4) Write a program that can reprogram the attached storage chip and store it in the one of my AT89C51RC chips.
5) Replace the 8031 with  AT89C51RC.

This results in the EA pin being used as ISP Enable pin. When it is asserted (default on 8031 boards) the AT89C51 ignores its internal Flash entirely and execute the code on the external ROM (or FRAM) chip. When the pin is deasserted (using a jumper wire) it boots into the onboard Flash, which includes a small program that updates the said external storage device.