We are planning a similar project, as I want an overhang in the tank approx. 12" into the tank from the side wall and approx. 3" below the water line. We work with polyurethane foam at work all of the time when making automotive parts, and the adhesion properties of this stuff is great. I've considered useing my tile saw and cut the rock flat, apply the white insulating foam from the can you can get at walmart (same stuff, isocyanate and polyol) to the flat rocks, and glue them directly to the glass. Eggcrate might be nice for some things, but in my experience with eggcrate in the industrial world, it's not UV-stable, and can get brittle over time when exposed to UV light. Again, I'm not experienced in the reef world, so I freely admit that I may be talking out my arse when speaking about the applications of eggcrate in a reef tank. I'm using schedule 80 pvc in my tank where I need a framework. Project you're working on looks good, though.
Kev