Filling the Internal Portion of a Clipped Object

This is a confusing question for me to even attempt to pose, but here goes:

I'm trying to find a way to display a filled cross-section of a mesh, which will eventually be my 3D terrain. As it stands, I'm using an orthographic camera set up with its clipping planes intersecting the terrain object (in the picture below, the horizontal box is the stand-in terrain and the camera is highlighted). What is displayed in the Game View is the small sliver of the top of the terrain object.

pic

What I'd like to end up with is the entire slice of the terrain object be visible as a solid fill - think of old cartoons showing cut-aways of underground with dinosaur bones and whatnot.

Here's the catch, though: I can't just create a custom object tailored to sit at the correct dimensions and angle because I'd like to have the camera rotating as part of my project. This means that whatever terrain I end up creating (hills, etc), I need to find a way to determine where the 'surface' sits, then find a way to render the area below it.

I hope my explanation wasn't too convoluted; please ask away for clarifications.

Thanks.

2: [URL=http://img339.imageshack.us/i/clipping.png/][IMG]http://img339.imageshack.us/img339/6236/clipping.png[/IMG][/URL] Uploaded with [URL=http://imageshack.us]ImageShack.us[/URL]