How to add clouds or fog to an existing map in GIMP?

[sambrookjm]

in Gimp 2.6 there is a filter Filter>Render>Clouds>Fog that produces a semitransparent random cloud layer.

Are you sure that's not a homemade script that you've got installed? I've got GIMP 2.6.5, and I don't have the Fog option in the Clouds submenu. I've got Difference Clouds, Plasma, and Solid Noise there, but no fog.

Granted, it would be easy enough to make a new layer on top of your map, render some random noise, and reduce the opacity somewhat (depending on how thick you wanted the fog). Maybe change the layer mode to overlay as well?