It is a bit difficult to adjust the slider for hide/reveal with the mouse and I've yet to find a need to have a % opacity in the fog. Some options that could improve things:
It would be nice if hide and reveal could be tied to the right and left mouse buttons with the square and paintbrush tools.
Holding shift or ctrl while left clicking will hide and only left click will reveal (realized that pan is tied to right button, so this might be better than above)
Hotkeys for switching hide <-> reveal.
Make the slider larger so it is easier to click and drag.
Support binary hide<->reveal instead of percent hidden/revealed.
It is a bit difficult to adjust the slider for hide/reveal with the mouse and I've yet to find a need to have a % opacity in the fog. Some options that could improve things: