Response to gross-fog density with different developers

Andrey. Film density curves start off shallow, that is, with little variation in density for a given exposure increase, and increase in slope as exposure and density increase, until they reach a more-or-less linear region where the density increase for a given exposure increase (gamma or contrast) is at a maximum.
Now, if we artificially push the shadow exposure further up the slope, then the contrast (tonal separation) will be increased, since some of the toe of the curve is now taken up by fog, and our real image detail is riding higher up the curve. Of course, it has to be a special type of fog, such as a regulated pre-exposure, or general flare in the camera.
I'm not sure if any chemically induced fogging would have the same effect, so in the context of this thread, you're right, it's not really relevant.
I think we should make a distinction between two types of fogging. Veiling fog, which is usually heavy and flattens contrast, and 'revealing fog' which actually improves the image by overcoming some of the toe inertia of the film.

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