Thanks! Actually it wasn't too difficult. It's all in C++ but the Adobe Illustrator SDK came with some examples that were pretty close to what I wanted already. Once I'd figured out the maths I had a prototype working in about a day.
The one thing left that I can't figure out is how to have the curvature comb update live as you drag the Bezier control points. Currently it only updates when you mouse up.
Does it run on multiple platforms, or only OSX?
It should run fine on Windows as well, but I haven't had a chance to test.