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.
I'm sure you could have replaced the chip containing the ROM and replace it with your own version. Things were a little bit more rough back then.