A bunch of nitpicks below. Haven't tried it yet, though :)

Is there a chance we could serve the novice user usecase of Meta+P and then *click* / choose the desired setup? As far as I can tell we only show "You now use $setup" but not "You can choose: [Clone, Left of, Internal only, External only, Right of]" (doesn't neccessarily have to be part of this patchset, obviously, just food for thought, and you know how obsessed I am with the Meta+P menu ;)

It has the future wayland problem that MG wants to limit which applications can access which interfaces; you're implicitly using a plasma specific one in Dialog to position the window, which would make kded need to be a "trusted application"

So whatever this solves, I think we might end up going back and having to fix plasmashell in the future.
(or move this into low level kwin, which is the only way you'll be able to show the right output identifier on mirrored screens)

I actually talked about this with @graesslin, and this is the solution we came up with. When we limit the number of processes that are allowed to talk specific protocols to the Wayland server (the plasma surfaces, for example, but also the output configuration protocol), we'll move this code into its own daemon and kwin will authorize that daemon. THat's basically the same approach we'll have to take with powerdevil. This patch does actually help it since we reduce the number of entry points for the OSD to one (just kded) instead of two (kded and kcm).

Bottom line: Considered it, it's part of the plan. :)

In D3598#67455, @sebas wrote:

protocol), we'll move this code into its own daemon and kwin will authorize that daemon. THat's basically the same approach we'll have to take with powerdevil.

so in the end the authorized process would be that separed daemon and not kded? (as kded can in turn run all sort of 3rd party stuff, would be better if itself is not an authorized process)

Correct, that's the idea.