The only way I’ve found is to make the global volumetric fog on the height fog actor be VERY low, basically invisible. Then pump the volumetric scattering intensity way up on lights or bring it out with a particle or mesh, with a Volume material applied that has a very high extinction.
It’d be nice if we could somehow cancel out volumetric fog in a certain area, but volume materials can only be additive so I’m not sure there is a way.