What you want at that point is to agree where to read and write common configuration. I'm not sure this is likely to happen either, as gnome has gsettings/dconf and kde has simple files and you will now awaken the plaintext vs efficiency dragon.
The current situation is just how it coincidentally turned out - there's just not always a clear direction on the free desktop (and that's even what many people like about it). If wlroots or something similar had existed earlier, then this probably would have turned out differently but c'est la vie.
edit: looks like it might already be provided via wl_output::scale [1] except you get the wrong effect that way. You want to tell the client what DPI they should render at, so that you can keep scaling at = 1
[1]: https://wayland.freedesktop.org/docs/html/apa.html#protocol-...