I was having trouble with the mono I was using to pull legs through soft bodies and hair bugs. I was also using mono on a small bobbin threader I made. The mono, at least to me, has a down side. It does seem to get brittle over a period of time and breaks at the worst times.
I found that a wire from a heart catheter is perfect. It is coated, so it will not leave any marks on the inside of the tube of the bobbin, no matter how many times it is used. If the coating is removed from the wire, you have a wire that is almost invisible but will not break when pulling rubber legs through a body. I thought maybe the wire being so thin, it might cut the legs. That did not happen.
Hmmm I was picturing you In your waders, and fishing vest, carrying an old stinky trout net into the operating room waiting for the doc to cut off some spare ends for your fly tying. Lol
PS if it is thin plastic coated wire, might it not make good legs that you could bend to shape?
I was having some pretty bizzare thoughts myself. Now that you mention using it for the legs themselves, it also could work for ultra body material and possibly some very sturdy dubbing brushes.