I haven't been active but recently looked at this interesting thread. I had done some experimenting with using fake stones on discs. The trouble with discs is that the varying density of stones does not cast as ring shadows on the planet, the disc seems to be a one-way object with impinging light and wants to only cast a single shadow. From this perspective you can't get "under" the rings and look up through them either. This is from a while back so I had to look in the file to see that I used a glass shader on the disc to get transparency. Trig functions make the ring structure as masks for the fake stones. Since I couldn't get proper shadows from the various stone densities I faked it with a few discs and soft shadows. See pic. I like Mr. Fraser's approach since he gets the explicit shadows on the planet and can get close up to the constituent particles. But I like fake stones because you don't have to populate, they are fast. By coincidence, I had recently thought to use a distance shader limited atmosphere for rings, but could not get the distance shaders to limit the atmosphere to a thin slice, and have had only primitive success with the cloud approach when that failed (was trying to apply the trig functions but it seems they don't map to the cloud slice the way they map to the discs).
As an aside, real planetary rings look different from top or bottom with respect to the sunlit side because different particle/chunk sizes have different forward and backward light scattering properties. -Bill