Here's an example of a new (to me) source of vector art for rendering on a polargraph: "sliced" 3D models.
I noticed that Slic3r (a program used to process 3D meshes for 3D printers) has a "slice to SVG" feature. -*lightbulb over my head*-
There are quite a few hoops to jump through to get a set of vectors that are useable, and the visible smudges on this drawing show that I should have jumped through a few more, but I'm very excited about the possibilities.
This is a rendering of a strange head scan, available as a free sample here: