After viewing Damian Nenow's presentation about "paths of hate", and being intrigued in his solution for clouds in 3DSMAX, I decided to have a go at mimicking his workflow in C4D. There are probably other ways to do this entirely in C4D without any 3rd part plugins, but this was the first thing that came to mind. I've since gone back and tried another method, but I still think that this works better.
*This scene requires Biomekk's Lumen 2 plugin in order to work.*
The original mesh of the cloud has been baked (GI, and some Chanlum SSS). This texture is then used by the Lumen plugin to control the color/luminance of cloned primitives which cover the cloud mesh. There is a fresnel falloff shader with some fusion blended noise in the alpha channel on the clone's material. This gives soft edges and a more believable cloud look.