At the risk of killing off a perfectly good thread with some actual scholarly research, the best history of "hootenanny" as a folk music event is an essay entitled " 'Hootenanny: The Word, its Content and Continuum" by Peter Tamony, which appeared in the July 1963 issue of the journal Western Folklore. It's been reprinted in several places, including The American Folk Scene: Dimensions of the Folksong Revival, edited by DeTurk and Poulin (Dell, 1967).

The first Hootenanny was advertised in the Washington New Dealer (July 25, 1940 p. 4). It promised "Dancing, Refreshments, Door Prizes, Uncertainty".

By 1943, they were a regular institution in Seattle. The New World (July 15, p.3) [The New Dealer became The New World earlier in 1943] reported that "The New World's famous Hootenannies will celebrate their third birthday [on July 31st]. The first Hootenanny was held in July, 1940 and the affair was such a success they have been held regularly since that time."

Regarding the choice of name, Tamony quotes a letter from Terry Pettus, who had been editor of The New Dealer/New World:

"It is true that I suggested Hootenanny. It came to mind as a result of the need for a designation for monthly events which would follow no particular format. I remembered that in my youth in Southern Indiana the word Hootenanny was used to designate a party which just seemed to happen as against being planned.

"These affairs were held in Polish Hall. The downstairs consisted of a large room with a bar and a small stage, plus the kitchen. The upstairs was a pretty good dance floor. Thus, depending on 'special attractions' etc., a Hootenanny could be a dance--a stage show--a concert by some visiting troubadour--a beer drinking, record lisening evening--or a combination of all or some of these ingredients. From this flowed the need for some all embracing designation."

Time and later Fortune quoted Guthrie's "hootin' Annie" story in 1946, but it seems safe to believe that Woody was not a reliable source.

Terry Pettus was born around 1900 (I remember meeting him in his eighties, around 1982), so the word must go back to his childhood at the turn of the Twentieth Century. The Dictionary of American Regional English offers five definitions that all come down to "dingus" or "thingamabob"--some kind of unspecified gadget.

The Bob Miller song, and the claim by Texas Jim Robertson (Sheriff Tex) probably pre-date the 1940 parties in Seattle. But it's pretty clear that it was indeed the New Deal/New World events that inspired Seeger and Guthrie. And the modern use of the word to describe a folk music gathering started with Terry Pettus.