the use of mirrors interests me too, and I have considered it too. I have an excellent refererence book, Baensch Aquarium Atlas 1, and I'll quote it's discussion of backgrounds to you....
" "Cute" decorations such and divers, seahorses and treasure chests are generally fround upon but the back glas can be made interesting and appealing. It should be coordinated with the rest of the tank. A blue seascape, for example, should not be used on a freshwater tank. The best back panels are made of durable printed plastic. Painting the glass, as is sometimes done, is not reccomended. Paint is difficult to remove. A homemade panel can be extremely effective and here is one idea:
Build a wood frame the exact dimensions of the tank. Attach a mirror to the frame, then spread a little glue over its surface in an abstract pattern. Sprinkle colored "sparkles" over the glue, leaving patches of mirror showing through. When it is dry attach the panel to the back of the tank. It will add visual excitement and an illusion of depth. You can also add strips of bamboo, dried grasses stones and such between the outside of the tank and the mirror"
I think this would be neat, but have yet to try it. Of course, Baensch doesn't describe exactly how to attach this to your tank....