Hi Lucio.
I have been playing with cloud masking on and off and was going to start on a similar project with the alps. Even the landsat data for the area still has orograhic clouds along one side of aridge through a significant portion of the image. The main problem I find is that as soon as you start masking the cloud fractal you run into problems with the edges smoothing out too much.
My next set of experiments in this area were twofold: 1) play around with order of nodes using blending shaders. e.g. fractal blends image or image blends fractal.. there are differences in the outcome but I haven't done enough testing to fully grasp it yet... 2) breaking up up the non-white/black portions of the image map output (i.e. after anti-aliasing and colour adjustments) with a fractal noise. Sounds simple enough, although the interactions betwen the two sets of fractals (breakup and cloud) complicate things a bit.
Hope this makes sense...
You're definitely heading in the right direction though