It is a very good idea about SRAM! Around 16KB of SRAM can fit on the die (Wafer.Space provides 20 mm^2 area per chip) - that could be useful for both fixing ol

It is a very good idea about SRAM! Around 16KB of SRAM can fit on the die (Wafer.Space provides 20 mm^2 area per chip) - that could be useful for both fixing old computers like ZX Spectrum and handy for turning Z80 into a tiny MCU. However Flash/EEPROM is currently out of the question. I don't know of any open-source versions available. That would have been a very serious undertaking on it's own!