Found it almost by accident while working on something entirely different, and maybe not exactly the same kind of mapping you would find, say, in 3D studio or Blender, but a fast and cheap way to test your mesh normal orientation.

Enough foreplay, here it is:

All the first part is just to generate some noise-displaced mesh, but all in all is very simple: plug your normals into the vertex color input (I cast them to colour right outside for speed purposes). It may also be nothing new, but I searched a bit without finding anything similar.