Pretty cool, eh? It's actually pretty easy to do. I'd love to try it with live rock, by making a base out of styrofoam, sticking on smal pieces and then siliconing on larger bits. The problem is that you can't have very large bits siliconed on, they just fall off - so you'd have to come up with some way of supporting them. I've heard of thin PVC tubes etc, drilled through.
It would help on my new set-up because the paint that I sprayed inside the overflow to hide the durso standpipe, came off within about 2 hours of getting salt water on it. I'd tested it and everything. Anyway you can now see straight through it.
I'm actually thinking about laminating a piece of card the same size and just slotting it in the overflow box. It will probably look like it's painted after there's a bit of algae grown over it.