A writable multi-cartridge for a VFX would require an EEPROM compatible with but significantly larger than the 28C256, and I haven't had a huge amount of luck finding any that are more than 1 Mbit (such as the Atmel AT28C010) which would correspond to 4 plain cartridges - and thus not a huge advantage over 4 separate cartridges. With a buffering SRAM (that the VFX reads&writes) and a microcontroller that shuffles the data to&from a much larger Flash memory, things are not limited by existing EEPROM sizes, and that also allows using serial flash memory chips that are quite compact. Or it would allow creating a cartridge with an ESP microcontroller that would allow WiFi access to the cartridge (which would be rather cool actually, but even more over-the-top) :)
// Christian