Comment by erik

There is one obscure product that actually did use the feature. The Sharp Famicom Titler (or Famicom Editor) was a full Famicom that could show an external video input behind the Famicom graphics.

I found this video that shows a Playstation running in the background of Super Mario Bros: https://youtu.be/TCsle-J9YzY?si=oyj_zZCKGionnzLu&t=423