This is exactly what I did. I made the ROM writable by soldering a CMOS ram chip across the ROM (was it 16KB?) and any write to the ROM would end up in the RAM

This is exactly what I did. I made the ROM writable by soldering a CMOS ram chip across the ROM (was it 16KB?) and any write to the ROM would end up in the RAM. A switch selected either the ROM or RAM to read from. On startup simply copy the ROM to the RAM and flip the switch. From there on you can read/write the original ROM and modify it to your hearts content.