Are you wanting the structure of the rock, or the bacterial benefits, or both?
Big problem is being able to support the weight of the rock... is you're primarily wanting the structure of the rock:
-make foam rocks! You can make them in whatever shape you want, if you mess up, simply make another one. They will cover over with algae, coral will grow on them, you can stick frags on toothpicks and stick them into the foam. They will be bouyant, but a strong magnet should be able to do it. Especially after you have some corals or whatever on it that will help weight it down a bit.
[B][U]Marineland 5G Corner Tank[/U][/B]
-(2) Red Sea nano-filters + stock filtration, (2) 50/50 10watt screw in CF's + LED actinics, mini-heater, Koralia Nano
-1.5" LS bed, ~5lbs LR
-Black Clown Pair, Porcelain Crab, several misc snails/crabs
-20+ misc zoo frags, shrooms, yuma, blasto and acan frags, green leather, echino frag