I'm hoping that @zamundaaa will add that to KWin, plasma-wayland-protocols & libkscreen
libraries/plasma-wayland-protocols!90 would be one approach to do that, which is nice and simple... but it also doesn't fit that well into the output config stuff, as it's not really a setting.
We could instead make a dimming protocol that allows you to create extension objects for the config outputs, but that feels like a lot of additional work for little benefit. If we could have dimming as a global thing instead of being per device, we could make it a separate protocol without any additional complexity.