I can't find out anywhere what you put on the bow hairs. It's just horse hair, like any other stringed instrument bow, which implies that I should use rosin, just like any other stringed instrument, but I'm really nervous, especially because I'm afraid of what the rosin dust will do to the leather, which is probably the scariest peice of the kokyu for me to damage. (I don't even know how to rehead a drum, let alone a kokyu.) The internet is not being useful. Even the one and only youtube video of someone playing it has been removed. Most of the pictures are small and don't show all the bits. And if I search on the kanji, I tend to get ehru.
So, I've decided I'm going to document the adventure and try to document as much as I can in English about what I learn about this instrument. I might have to go look up some instrument scholars at berklee or something.