I've just poured flourite behind hardscape, with the substrate acting as retaining walls. It works fairly well, but untill you get alot of plant roots, it does tend to try and settle out. You can create a tiered effect simply by allowing the plants behind the hardscape to grow taller also.
Some carpet plants don't lend themselves to this style of trimming though as they tend to grow very dense. The taller the tank, the more difficult it is to get that same effect since you'll have to make your raised portions taller to make them seem as high in the overall "scape".
There's 25 lbs of driftwood in my tank and a 6" difference between the low spots and the tallest parts of the substrate, and it's barely noticeable now that the plants have covered substrate.