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The idea was born with connection of Ubuntu Guest Mode and KScreeLocker media player. Guest mode is insecure, but we can extend KScreenLocker media player idea to playing also slideshow of images and videos, but on login screen.

KDE should create directory in /usr/share/public, where all user can write, but nobody could delete/edit other user files. If SDDM detects some files in this directory, media controls would appear.

Why this idea? User often uses computer to playing videos or movies and for anything else, so by realizing this idea we will allow users to play media without providing password. We can turn on computer for our friend or children and didn't worry about they will do something wrong.

EDIT: I don't know you can realize this idea in secure way, but hope you can do it.

I see this idea is partially realized on screenlocker. Thanks!