Is it possible to add background music (with or without video) to the login screen on a Mac? I've never seen this question asked before, and provided that Terminal exists I don't see why this wouldn't be possible.

Does anyone know whether this is, or how they would make this possible?
That is a nice idea !
I have just seen that you can play sound with the program afplay, you can look how to use it here : https://osxdaily.com/2009/10/27/play-mp3s-and-other-music-from-the-command-line/ .
One way to do it could be at startup at the first login screen, you can look how to do it here : https://superuser.com/questions/245713/whats-the-easiest-way-to-have-a-script-run-at-boot-time-in-os-x .