Hi,
Because I cannot work on the shaders at the moment I thought I’d post my results so far on a shader that models a 3D object (venetian blinds in this case) with just a 2D shader. The technique is in a sense related to carpaint and toon shaders as it depends mainly on reacting on the angle of incidence. Anyway, details in this blog article:
http://blenderthings.blogspot.nl/2013/05/working-with-incidence-vector-in-osl.html
A sample that shows how the apparent width of the blinds changes with the view angle while the shadows are correctly rendered is shown in this clip