yeah, I've been avoiding that direction myself. I think the full tank, siliconed in, shelf with water under it is problematic in it's self. you'll never be able to really remove the shelf, without dumping half your beach in the water... the space under the shelf will likely become a trap for algae and debris, and you won't be able to get under it to clean.
I do like the beach idea a lot though, and will probably build something at some point. the key is to think about what you're going to keep in it first, and make it for them. you'd want a different beach for a red claw crab than you would for mudskippers, since the crabs need to burrow, but the skippers don't. you'd want a different beach all together for something like hatchets...
If I was building a beach now, with no limits besides space, I'd use a 30 or 40 breeder tank. I'd first lay a plumbing system on the floor of the tank, that allows me to pump water under the beach, then I'd stack some fairly large, but flatish rocks around the foot of the beach area, and fill in between the rocks with some sort of bioballs... like 1/2" ceramic balls or something, then I'd start adding sand... all the sand for the tank, but I'd add it all from the top of the beach, and would toss in a large rock here or there as I see how it's spreading out. I'd probably be trickling water in as well, to start raising the waterline. this process, I think, would create something fairly natural looking. some small, strategically placed, rooting plants would help with erosion, but for the most part, I'd let it do it's thing. sand moves. over time, you'd maybe pour a cup of sand on the top of the beach every now and then... and let it erode....