The opening line of the song mentions Mr. Kite. Shortly afterward (3rd line, I believe), is the line "The Hendersons will all be there", with the context implying that they are performers. So now we have a Mr. K and a Mr. H--there are your "Messers K and H". If I remember correctly, John Lennon was quoted as saying what Callie mentioned about the lyrics coming from a poster. I would have to think the author of the book you read, Sourdough, made a gigantic assumption, but a very incorrect one.
Incidentally, I have always heard that the "Spruce Goose" was the brainchild (and dismal failure) of Howard Hughes, and have never before heard Henry Kaiser's name associated with it. Am I ignorant about Kaiser's role in this, or is this author making all this stuff up?