I didn't just spend the weekend gardening.
I also learned a couple of things.
For example: toothy critter wire doesn't like being wrapped around a 1/4" diameter capstan. Not even the 49-strand, "super-flexible" stuff.
Modern deep-sea fishing line, on the other hand, is thin and flexible and wraps around the capstan nicely.
Trouble is, it's made super-slick... which is fine for normal fishing-line applications, but leads to a notable lack of traction on the capstan. Well, it might work better in the real contraption, with tension being properly maintained. Maybe.
I think I need a more elaborate test fixture... and maybe also some other options in the high-strength string division. Probably also a larger capstan and another stage of reduction gearing.