alt Hacker NewsThere 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
The EXT pins only allow setting the backdrop color to one of the palette values in the master PPU's color palette, so the Titler can't use that functionality to superimpose graphics over arbitrary video. It actually uses an RGB PPU which doesn't even have the EXT pins. Instead, it uses a separate video encoder chip for the video superimposing feature. https://www.nesdev.org/wiki/X0858CE