Speaking of weird chips from that era, Amstrad CPC464's sold in Spain came with an extra 8KB RAM on board that was not connected to anywhere. It was put there only to avoid import tax for computers with 64KB RAM or less.
I've rarely used the C128 mode in VICE, but according to the manual[1], x128 emulates a z80 too. It would be kind of cool to develop in linux and then "deploy" the code to x128's z80 emulator as a "production" environment.
IIRC the Z80 would be the first processor to wake up, too, and would retain control CP/M were being used but would shift control over to the other processor if either native or C64 mode were being used.
Even as late as the Amiga you got stuff like that. To this day you can probably find a PPC based accelerator for all models.
I guess the close to the metal, with little to no memory protection barriers, made these kinds of things easy to implement.
That said, some early GPUs were somewhat similar.
The 3DFX cards used pass through VGA cabling. And i personally owned a PowerVR card that wrote rendered frames back to the graphics card over the PCI buss.
