Cute idea. Care to suggest a URL of two about pluggable ROMs for interpreters?
There's some high-level stuff here http://beebwiki.mdfs.net/Paged_ROM (see "Languages") and I have asked my BBC Micro developer friends to send me some more useful docs, which I'll forward to you as soon as I get them.
Edit: Actually that page has some pretty low-level stuff on it as well, which may keep you busy for a while.