Sorry Bruce, but I have no idea if there is any documentation about any link between the Dai Nihon Butokukai and Korean Kumdo. I really don't know much about the DNB other than what little I've read. I was mainly trying to clarify that the linage chart is explained, although not very well, and it is not something the author of the book created, but rather something of more historical value.
I do know that kendo is quite popular in Korea. A good friend of mine who lived in Korea during the Japanese occupation often talks about how he learned kendo from the soldiers stationed in his hometown. As for official links, information sharing, etc., I guess we'll have to see if anyone else has something more on that subject.