You don't need soil, all you need is something on the bottom that acts like a sponge to keep the humidity up. Gravel will work, but gravel with coco fiber on top is the best in my opinion. Moss won't grow very well on gravel (if at all). Most plants won't do all that great either.
It's really easy to make a pretty nice looking terrarium with some gravel, coco fiber, and a few easy plants.
The Houston Zoo's dart tank is all gravel for substrate with some water features made from stone and what appears to be some sort of mortar mix. Like David mentioned there is Java moss growing in the moist areas. They have a large water feature on the left and a river that drains to a lake on the right. There are only a couple species of plants in there and they are huge and rooted in the soil. Don't know about the other mosses, although it might be worth experimenting since they do grow on rocks in the wild or on compacted slightly acidic soil.
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