As title says, it would be nice for modpack devs to be able to play local files from the .minecraft folder at least, to not rely on a modpack player to have an internet connection when they play the modpack. For example to be able to pack a small video in, let's say, "config/videos/" folder and then play it either with playvideo command or a tv.

Have you tried minecraft://, file:// or local://?

Ah, so that's the formatting, in that case this feature already exists and the command /playvideo local://config/video.mp4 would play a video from a config folder named video.mp4

Thanks for the help, this is an amazing mod!

I tested it and the best way to get the URL is to drag the file to your browser as a new tab, then copy the URL that says "file:///X://video.mp4" and paste it into the command like this:
/playvideo @s 10 file:///X:/<videopath>/video.mp4

That is the best way to get the full pathname if you don't have any third party tools that can copy the path for you, like Everything Search and Directory Opus. However, I couldn't get it to work on the mod's included TV. Pasting in the same link does not function, although a YouTube video does.

Can you play local videos on a TV and local music on a Radio (blocks you place in-game)?