cool jan, must have missed that thread.
the problem with this trick to fake caves is that you have to decide whether you want detailed surfaces inside the cave or if you want it to look great from outside. what i tried now was taking two fake stone shaders, both with individual redirection, after that another redirect shader with bigger displacement. still good surface inside, from the outside, well, still too much like fake stones i think, but you have to be careful about the last redirect shaders displacement, or your cave will be gone.
only low quality renders, sorry about that, but i was doing something else when i looked up this thread again, and i promised a cave
so here it is, still very basic