Mike McCarey made a similar cake and while I can't find the info, I know that I saw somewhere how he did it.
If I remember correctly, the bulky part of the "mountain" is cake. There is a thin copper tube mounted inside to the board that comes out of the mountain and creates the swirl. He might have actually used even thinner wire attached to the tube as the swirl got thinner. Then he covered the tube/wire in modeling chocolate to get the swirl and then covered the entire mountain and swirl.
I may have dreamed this up but I am pretty sure I did actually see how this was done.
I have never worked with modeling chocolate before, it sounds like that would be a great idea! I have used rice krispies a couple of times. I made this dinosaur cake, the head and tail are rice krispies. I shaped them and then allowed them to dry. I then put a generous coat of BC on them and covered in fondant. I think it would hold its shape well, I am wondering if you would be able to to use some sort of tool to act as sandpaper maybe the finest side of a grater in order to get the rice krispies a smoother texture (an experiment waiting to happen!) to do that entire mountain in rice krispies would be a lot, but I am not sure about trying to do cake with the rice krispies on top?