One idea I had was to do it the way people make stone facades for Halloween displays. Basically, screw 1/4" plywood panels onto the wall (so they're removable), and then glue onto them "stone" slabs made from foamboard. The foam can be cut and shaped pretty easily with the right tools, and can be roughened up on the surface with spray-paints (they "eat" the foam to some degree). This keeps the panels light-weight and removable, and they can still be painted with latex paints to look very real.