Here is a proof of concept python script

First, you need to have the python-rpi.gpio and omxplayer packages installed

sudo apt-get install python-rpi.gpio omxplayer

• check that your configuration match the script settings and edit accordingly,
  select a short video clip (like this one) and edit the line

  VIDEO_FILE = '/home/pi/video/better-than-tv-ads.mp4' 

• start the script :

  sudo ./switchdemo.py

• this should select the STB output
• press button 1 on your switch remote
• your video clip should start playing
• at the end of your video clip, the script will switch back to your STB input

The next step should be to integrate this script with Kodi to have a nicer user experience.