So, Cebas Final Render has its very clever "volume solid" method of creating 3d objects from 2d geometry at render time, but neither Mental Ray or Vray have an equivalent. I've seen a couple of topics on forums over the years regarding using opacity maps and such to "fake" the look of geometry, but the conclusion is often the same; it doesn't look good enough.
There is however a method of creating "solid" geometry with displacement using just about any render engine, though my choice is VRay because it has quite a lot of control when it comes to this matter. It is quite a difficult process to explain in words, so I will try to use images where possible to explain the concept.
The usual way of creating complex 3d geometry would be to model it, or to fake it with bump/opacity maps - the problem with the former being that it could potentially be very time consuming, and the latter whilst being quicker will never look as good as having the actual geometry.
Here are some comparisons:
Bump map & opacity map:
Plain ol' displacement:
Displacement can make a considerable difference, but has the disadvantage of it only working in one direction, so when you go 'round the back of the mesh, it looks inverted. There is however a way around this, and that is to create a duplicate object with flipped normals and hey-presto! With displacement on both surfaces you now have a 3D object. It will require quite a lot of tweaking and so on to make sure the "middle" parts meet up without leaving any gaps, but when it works it works very, very well.
This method enables you to create fast, complex 3D geometry and is only limited by the fact that you need half decent maps & UV mapping to begin with. I'm sure there are dozens of ways in which it could be used, if not just for fun. It will never replace the quality that can be achieved by modelling the actual geometry, but is far less time consuming and is very easy to set up. I'd love to see any attempts/successes that you have with this technique.