Personally I would find it slightly mad to fly above a fog bank in a SEP whether it was legal or not.
I flew across the channel and never saw the water - a bank of fog from coast to coast.
I didn't consider this "slightly mad" as I was scraping along the bottom of CAS and (in theory) could have made one side or the other if the fan stopped. And as I could see the coast on both sides throughout it was legal whichever set of rules you use.