The best thing would just be if Epic would allow to tweak the heightfog locally via post volumes! This way you could have 1 heightfog actor but many different settings for it based on post volumes for different areas (and it would be awesome to be able to apply a material to the fog to get some animation going on in there^^). I remember that we actually integrated something like this into Unreal 3 back in the days when we worked on Spec Ops, so this shouldnt be to crazy to reimplement into Unreal 4. But then…I am no graphics programmer^^
Cheers!