It's funny you should ask this now, Daniel (who runs the site you found the ROMs and schematic on) and Phil (another 8-bit enthusiast) recently produced an EEPROM-based PCB for exactly this purpose. You can find out more about it on the system.cfg forum (in French).
Daniel has many other Thomson hardware projects, including an SD card reader and associated ...
As an alternative to your project, you can use The Mégarom T.2, it's a MEMO7 Cartridge for Thomson T9000, TO7, TO7/70, TO8, TO8D, TO9, TO9+.
It contains 70 softwares.
Another way is to use the SDDRIVE that is connected to the extension port of the TO7/70. It uses an SD card to emulate a floppy disk drive.