The limitations abound, but:

1. Function based texture applied to all objects but which doesn't affect
the background. E.g. function(sqrt(x*x+y*y+z*z)) appropriately translated
and normalized to taste. This method will fall flat with reflections and so
forth.

2. Vahur Krouverk's Povman may give more access to the ray length
information, including reflections, for shader writing. Not sure though.

3. A questionable alternative to not affecting the background color with
fog, is to render a dual colored fog which is, say white, in the foreground
and cyan in the extreme distance, effectively creating a different colored
background suitable as a mask. This requires two fog statements: foreground
colored fog and backgound fog set to extreme distance with a
subtractive/addative color which will result in the desired background
color when applied to the forground color.

Regarding media fog, you should be able to duplicate colored fog using
emission media (fog color) and absorption media (white). You may also have
to set the absorption intensity to about 10x that of the emission media.

Abe

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