I cannot find a module which does exactly what you want but maybe this module is a good way to start.
I don’t know how it works exactly because I don’t get how the omxplayer shows up but you can try this out since it plays videos what you require.
The range of a PIR sensor maybe is too wide. I expect it detects anyone as soon as he enters the room so the adverts disappear before someone can have a look on it. I have a HC-SR501 for my own mirror and it reacts relatively fast.
I estimate a ultrasonic sensor is better for this task. I think of the HC-SR04 which just detects objects directly in front of it (max. distance ~ 3 meters). As long as no one stand in front of it, a mirror will be useless anyway.
I hope I could helped you a little bit and could give you a push in the right direction.
Edit: recently found this: https://forum.magicmirror.builders/topic/2094/youtube-embedded-youtube-on-your-magicmirror which may also help you.