While finding material for the previous post I stumbled across the following interesting paper.
W. Wang, B. Jüttler, D. Zheng, and Y. Liu, “Computation of rotation minimizing frames,” ACM Transactions on Graphics, vol. 27, no. 1, pp. 1-18, Mar. 2008.
It describes a technique similar to the use of Frenet Frames, but which avoids the nasty twisting that can occur at curvature inflection points. I’ve not looked in to the details yet, but rotation minimizing frames might possibly be worth considering for integration into Functy if it gives a better visualisation.