This is by no means the answer, but I really think the best approach to this is to write (or find) a custom shader. I anticipate this by saying that writing shaders is NOT a children's game: it can become incredibly complex because shader programming languages are relatively low and rely heavily on knowledge of esoteric geometry / mathematics.
After some tracking, it looks like you will need a shader that achieves something called Sub-Surface Scattering (known in the conversation as SSS) - this is essentially the behavior of light through a translucent object based on its thickness (and some other properties that I I will not delve into):
To achieve the lithophane effect, you need to create a “thickness map” that will be matched to your mesh of objects, and then the user shader will properly diffuse light through this map to create an effect similar to what you want.
I learned all this by going through a fundamentally simple presentation made by a leading Rendering programmer in DICE games. Here is an example slide from a presentation:
This shader creates an effect similar to this, depending on the thickness, in real time:
If you are serious about achieving this effect, I seriously recommend doing some reading on Cg or GLSL shader programming. Personally, I like Cg because I was interested in learning about its compatibility with Unity3D. For this reason, I found an excellent resource for learning Cg, namely wikibook on this subject . It is (somewhat) characteristic of Oneness, but the principles will be sound.
In any case, I wish you the best of luck on this. Hope this helps!
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