I tried it
but there seems to be a fundamental limit to the size of p3d objects.
But in any case, I think I got it
I've basically sequestered Weather 1 (clear sky) and given it a gray sky texture. I chose that one because of 2 things mainly:
1. Foggy days are generally mild days, not stormy, so it made sense to use that "end" of the weather spectrum.
2. The Overcast value ranges from 0 to 1, and the next weather "up" (very clear sky) comes into effect at Overcast value 0.07, so it seems to me that "clear sky" is actually quite a rare occurrence
Anyway, I got a test mission with various triggers that invoke different overcast values, and different fog value. And, I can *apparently* dynamically make the sky gray for very foggy situations
Also, I'm going to try & make an 8th weather level, so that the clear sky texture isn't actually lost. Make it active at Overcast > 0.01 or something, see how that goes.
Almost there