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In KDE4, there's possibility to add widget onto screen look. As far as I known, this possibility was reverted due to security reason.

Why doesn't use solution using by flatpak (I don't remember name) to setup sandbox and plasmawindowed? On Wayland this could be secure. Of course - you need a way to set info in manifest that this plasmoid could been used on lock screen. I think - due to efforts to makes KDE flaptak platform worked - you will have people, who could implemented this sandbox.

At the very least, the lock screen themes can be customized using QML. There have been a few topics discussing how to do this (in particular setting up video backgrounds). I don't know of the possibility of using (any) plasmoids, though.
I would be interested in getting my calendar items on there (with the possibility to get the room information so that I can see where the meeting I'm missing is supposed to be held in directly from the lock screen.