i was thinking of some crude form of OS, if it was loaded (as some masive cart) with other carts stuck into the space what is't used by the OS program it could load these, but it would need to know its vectors and the loaded cart's vectors, as well as being able to store a program written in it- basic interpreter program perhaps, the cart could use sram (or whatever usb pen drives use to store files on),
i suppose they is't any such mapper capable of this though

accessing vram outside of $2006/7 would be good