For above the roof, and below it, your could try adding fake stone shades limited to low slopes as a child of the ceiling displacement limited to the height above the cave roof and the height below it. This can place rocks in those areas. Then you can work around smoothing those rocks above the cave to make the landscape more feasable, and the rocks below can be sharpened and shaped as required.
To help with the wet rocks you can throw a reflective shader on that darker texture to help, but that would slow down the render and could crash terragen entirely during the render process ;/
For a fake waterfall, add reflective, and water shaders to a bule 'verticle' or semi verticle and displaced "wall" of rock, and just add a low level lambert shader or a default shader. You can use a multishader to mix the colors for that "wall" with the reflective and shaping shaders to make it look as liquid as possible. Probably it'll need to wait till Planetside gives us transulency/transparentcy before it'd look convincing though.
