On the pumice, look carefully through the rocks, many times you can fins some that have black veining through them. While these black areas are harder to drill through, they are not impossible.
The GWAPA meeting host has a really nice tall tank, and wanted to build a wall of rock. He was going to use styro, but I sugested he make a baseplate of plexiglass, and glue PVC pipes vertically on it. The you can cut the rocks any way you like, drill a hole, and slide the rock over a pipe. With a little care and adjustment you can build a really tall wall of rock in the back, add very little weight to the tank, and not have to worry about anything getting undermined and falling.
One more option is to make flat surfaces and silicone the rocks directlly to the back wall, but if I ever do it to my tanks again Rodney will kill me. <grin>