Is it possible to visualize ray length info as a black and white map? Prompted by my previous question, Is it possible to see the output of a node group like this one: (created from this tutorial on blender cookie)





as a easy to visualize (and quick to render) black and white map like this?:



Simply connecting the output of the group to an emission shader does not let rays through the object, so the output is white:
Connecting the emission shader directly, as you suggested won't work indeed, but how about mix of transparent and emission depending on Geometry->Backfacing? Should work.

Edit:
Works for me: