Hi Mike. Was the term Kokyu used prior to O'Sensei in martial arts?
Yes and it's used in other Japanese arts, also, and I'll guarantee you that the karate and koryu styles that do use the term didn't get it from Aikido. If nothing else, a commonly known (in the Aikido world) example would be the basic jin/kokyu things that Ikeda Sensei got from Ushiro (karate). Then too, other terms were also used to describe the various ki skills in Japanese martial-arts (Reiki pops to mind as another term for Kokyu).
Frankly, given the fact that Ueshiba came from a very traditional culture, I don't think that there would be much tolerance for people using traditional terms in any way that they see fit. It just wouldn't fly; there wasn't that "words mean what I want them to mean", etc., that you see in a lot of the current western counter-culture.