Would not the black of the lith still let some light in on a long, long exposure and add a bit of fogging? Also, will the clear spot soften the light by acting like a filter made of film? However, it's an interesting idea and could provide an uniformity to pinhole photography and lith film has an incredible resolution. Meanwhile I'll stick to Captain Beefheart singing about his Clear Spot.
http://www.youtube.com/watch?v=RqYfmfEqi20