You could place large mesh over the map with many vertices and quades in it like a grid over your map when the guy/unit picks up you remove the face of the fog mesh where he picked. Now the cam will only see through where your guys have been..
To make it war craft style.. You could start the fog mesh at an alpha of like .8 you can still see through a little, to keep from seeing the enemy units they too would pick up, and if they hit the fog layer they alpha to 0, otherwise alpha to normal.
To make it war craft style.. You could start the fog mesh at an alpha of like .8 you can still see through a little, to keep from seeing the enemy units they too would pick up, and if they hit the fog layer they alpha to 0, otherwise alpha to normal.