My first thought, not being good with math, or spatial thinking, was just to postprocess the rendered depth map somehow, look for sharp dropoffs, and then do some kind of intersection between the lines from the map and the model
> postprocess the rendered depth map somehow, look for sharp dropoffs
This is fine, if you want a simple ~solid stroke. A simple edge detection kernel filter (e.g. Sobel) on the depth and/or normal map is the basis of most outline-like things you see (in games at least) today. This is useful, but it's not an accurate representation of the occluding contour, and it's in an unhelpful form for further processing (kind of like a raster vs. vector image).
