@Manuel I know that you can use tR's internal memory mapper in 8KB blocks trough firmware slot. How it works exactly, I don't remember... I've seen some documentation about this feature (made by Bifi?) but unfortunately I lost it due to mailbox corruption.
About 20 years ago I made TRLOAD, that is a ROM loader, similar to Megamu, for turbo R, with save states (added by Okei).
It uses Panasonic mapper that is similar to a megaROM mapper, but can access the whole address range #0000-#FFFF in 8K banks. Also it supports 9 bits bank numbers. Depending on the bank number range, it can access firmware, BIOS, RAM...
Yes, sounds about what I remember... How ever one thing that I wonder is that how much mapper RAM you can actually point? I mean... 9bit*8K is only 4096K of memory and even the mapper alone can be this big in theory...