Sure there is. It is called what is “visible to the naked eye”. And this is in the field of optics, I suppose if we are to classify it.
The bottom line is that it it does not work so it is not a valid solution to me. Maybe in theory it might seem logical but it falls apart immediately on application. Because I think it has more to do with optics than with the psychics of gasses and particles alone. Just to take a stab at it.
But however you want to spin it. It does not work.
I’d rather have a cheap fake in this case rather than something that attempts to be real (whatever that may actually be) but fails.
The scene they sent me as a solution, did not work! If anything it was a better scene to send them to describe the problem. And they did not have an answer. Because there is no answer. This is fog. It does not work to fake or even produce the real effect of particles illuminated by light in a controlled way.
So the answer is, there is no solution for volume lights and shadows in Vray.
The solution is to render in Mental Ray and fake it with Volume Lights.
Or as you say, do it in post.
Hence my plea. Just write a shader for a light. Done.