Thanks 
Yes. Small steps, but eventually they will get one there…
That is sure to come. Im just still resisting the temptation to keep lighting build times within reasonable bounds.
For the same reason, I already considered to move the landscapes to a different level and unload it for build iterations on the meshes.
If I decide to do that, the Skybox also would go there (and fog and such).
There seems to be no other way to explicitly exclude expensive items from lightmass calculations.
At the moment Im unwrapping meshes. When that gets all to boring, I may tackle that