I've designed a hollow part with curving surfaces that I want to slice in
half, approximately, but with a draft angle so that one side overlaps the
other a little. I'm doing this by creating a solid which I can intersect
with one side and difference with the other. My current approach for
creating the angle is to create a 3d hull between two this extrusions of
parallel projections, one of the inside of the part, the other of the
outside of the part. This is pretty good for convex regions where the walls
are pretty similar in thickness, but fails where the profile is concave.

I'm wondering if there are techniques for turning a projection into a path
along which I could compute values (e.g. turn it into a set of 2D points).
And similarly, for being able to determine the slope (gradient) of a
surface at such a point.

The short answer is: no, not within OpenSCAD.

The longer one: the only way to currently gets hands on coordinates is to
export your design into one of the supported output formats, change it with
an editor or parse and output it with some external program into a list in
SCAD format and reimport it to your SCAD file via include<> or use<>

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