Hey,
You guys are definitely on the right track.
It is defined by zones in the BSP.
...and, yes, obviously Swamp Fox knows how to do it.
I have put the "real fog" from 343GS into a map, bitmaps and all (just fog, no bugs), and I know where to implement the fog tag in the scenario, but unfortunately I can't make it show up anywhere because I don't yet know how to define a zone for it in the BSP.
If I make any progress I'll let you guys know.