Teeny-tiny update, a test render of my planet with it's broken shell to check the texturing. The nebula is by Matt Tarling, the planet and shell interior textures were made using the tutorial here.
I'm thinking about some debris and clouds around the planet using particles and hypervoxels, I'll have to do more experimenting. I'm also interested in rendering out some 3D nebulas for this project. I don't need them to be in 3D at the moment, but I haven't had much luck hand-painting them in Photoshop, and the results I've seen in a couple threads on the Newtek forums
are very promising. No details about what they actually did to get those results, so it'll be back to experimenting for me.
ETA: Early volumetric light nebula experiments are promising at thumbnail sizes, less so at HD resolution.