I have been thinking about this "ruin" for a few days. I am wondering if a null function could be used, merging with the default image color, and routing in a power fractal displace. I am trying this method now on a statue, PF texture and color only. I have a "perfect' castle I am going to try this on too. I believe I saw this method in the Materials library shares, "bronze eaten away" file by fleetwood.
Great images by the way!