For folks wondering how I did the Blender sketch art thing. I have attached images of the compositor node setup I use, the images/render layers used and the result.
Basically, I use a shaded grey render as a mask, combined with a colour ramp to determine at what brightness range to mix in which pattern, then overlay the lines.
I guess I'd need to do a step by step (video?) once I've settled on a final look. Still very much in progress. :)