I used magnets to make a Pukani rock corner shelf for my new upcoming build.
The magnets are plastic coated (from K&J Magnetics) and have worked well for me with magnetic frag racks.
The magnets are epoxied into small recesses in the rock; one on each side of the rock.
View of the rock shelf inside the tank.
<a href="http://s32.photobucket.com/user/jason_langer/media/Fluval%20M60%20Reef/Pukani%20Shelf/Shelf-MountedonRight.jpg.html" target="_blank"><img src="http://i32.photobucket.com/albums/d4/jason_langer/Fluval%20M60%20Reef/Pukani%20Shelf/Shelf-MountedonRight.jpg" border="0" alt="Shelf - Mounted on Right photo Shelf-MountedonRight.jpg"/></a>
Views from outside of the tank.
<a href="http://s32.photobucket.com/user/jason_langer/media/Fluval%20M60%20Reef/Pukani%20Shelf/Shlef-OutsideView.jpg.html" target="_blank"><img src="http://i32.photobucket.com/albums/d4/jason_langer/Fluval%20M60%20Reef/Pukani%20Shelf/Shlef-OutsideView.jpg" border="0" alt="Shelf - Outside View photo Shlef-OutsideView.jpg"/></a>
<a href="http://s32.photobucket.com/user/jason_langer/media/Fluval%20M60%20Reef/Pukani%20Shelf/MagnetDetail-Outside.jpg.html" target="_blank"><img src="http://i32.photobucket.com/albums/d4/jason_langer/Fluval%20M60%20Reef/Pukani%20Shelf/MagnetDetail-Outside.jpg" border="0" alt="Shelf - Magnet Detail photo MagnetDetail-Outside.jpg"/></a>
Using these magnets may hold the rock work in place depending on the strength of magnet you use.
But the stronger the magnets, the tougher it may be to break the attraction bond once attached inside the tank.