I would completely drop the idea of banking substrate and go with rocks. Rocks can be covered with plants just as well as substrate can and definitely make a great cliff.
I would be careful with rocks too! Mr. Amano had to change his plans during the making of this particular scape. http://youtu.be/_-Ex5RjPG3k
If I wanted to make a cliff in a smaller tank I would go with something like this. Flat rocks would be easier than the cobbles I use and I probably could make a less deep structure to save planting room behind the cliff.
Then I might fill the back with tubes of lava rock or even loose rock and fill in with the main substrate around that if my plan included deep substrate at the rear that is.