Render a plane seen from the side as line?

[EgonOlsen]

I fail to see the relation between the approach in the article and your problem. What they do, is to setup two shaders and define clipping planes in the shaders in a way that one part is drawn with one shader and the other one with the other shader. That wouldn't help here, I think. You can't "fill" something in between two objects with a shader unless there is actual geometry there that can be "shaded".