Ernie,
when I Google PEEK wire, I see links to Magnet wire (this has become VERY popular with the Qi wireless charging systems) that are so popular now - I see links to granger and Zoro for low prices, but 90+ feet of it...
I was wondering if music wire or memory wire would perform that same function, but there may be other considerations (abrasion of the mating member, "Set" in the wire etc.)
Finally, since these are essentially "Springs" is there an alternate product (silicone, PET, 3-D printed spring, etc.) material available today, that wasn't available in the 1300' -1400's that you are willing to use in place of the boar's hair? Obviously this may require a re-engineering of the keyboard, but it could be fun to develop an alternate solution...