Chris - the oldest extant densho of Toda-ha Buko-ryu dates back to the mid-1800's. We have a set entitled 鏁鎌合気之事 Kusarigama Aiki no Koto. It is very possible that the name is older, because this was from the 14th generation of the ryu. But it's also possible that the school was a family tradition, previously, taught by word-of-mouth, without documentation.
I can't recall the source, but I read somewhere that the original usage was in a text on swordsmanship and it did not imply a superior power at all, but more like a stalemate situation.
Does that sound at all familiar?