Tai no henka is excellent for developing the forearms, especially doing it with gorilla uke`s that hold on tight
. Bokuto suburi also develops the same muscles well.
As for the "hair cuffs", I `ve seen this and have my own set. A long time ago I was visiting an acupuncture sensei here in Kobe (where I am this week) he was also a chiropractor and 7th dan karateka. He told me that the body develops hair in order to protect itself, given the amount of katatedori I`ve experienced in the last 14 years, I can well believe that, just glad I don`t have anyone grabbing my back