I developed a prototype for a c128 multi function rom cartridge, you can choose which back of eprom you want to run and each slot is a cartridge version of a popular c128 internal function rom.
The code of every single function rom was adapted and hacked to run off external function rom aka cartridge.
This cartridge also includes the Basic 8 which from what I recall I never saw running off a cartridge.
For any info just contact me at: labs@codingkoala.com