Plasma wallpaper plugins are, well, for Plasma.

When it comes to wallpapers, you could do a similar trick on X11 DEs by putting it onto the root window (with a tool like xwinwrap) and on Wayland DEs that support layer-shell (with a tool like windowtolayer). I'm not aware of screen lockers that do something like that, but you could always write your own one.

Right, but I hoped it would work as a standalone Qt app.

Yeah, I've used xwinwrap before, but am lost on Wayland. I'll look into windowtolayer, thanks. I'd rather not have to write this myself...