> E.g. Commodore designs "borrowed" this from bigger computer systems when they put fully programmable 6502 based computers in their external floppy drives for their 8-bit line (you can load code into the memory of the 1541 floppy drive and execute it)

Yup Gwern mentions it in TFA. I had a Commodore 128 (well I still have it and it's still working!): Z80 chip in addition to the MOS ones (not just the CPU but the soundchip too).

I'd say though that back then none of these chips were added due to committees being infiltrated by three letters agencies or their chinese equivalent with the goal of introducing backdoors.