I thought about it a bit, and I think the simplest way to move forward would be to have a small Wayland protocol that powerdevil can use to expose the brightness devices to KWin, like https://invent.kde.org/libraries/plasma-wayland-protocols/-/blob/ef1041fca72bf614b946c129fc819df9925a8da0/src/protocols/kde-external-brightness-v1.xml
For all non-KWin brightness detectors, if that protocol is available, powerdevil would just not use them directly but only expose them through that protocol