No offense to johnross or Peter Tamony and his work of research, which I've not been able to read yet, but in my brief research about the word online I found this link http://folkarchive.de/political.html that describes a song called, "The Hootenanny Song" by Miller's Bullfrog Entertainers. According to this reference Bob Miller recorded this song in NYC, NY in 1929. Apparently it has some political reference to the message as it's included in a compilation of political songs. I'm not familiar with this rendition. This seems to predate anything already presented both in this thread and in another of the same subject where we all had a good go round about this same subject.
In that thread I was more convinced that Texas Jim Lewis might have been the first to use the term as a musical reference to his washboard gizmo that he called Hootenanny Annie. He had been a cowboy performer in the western swing genre and had recorded his music as far back as the 1930s. His Hootnennay Song was even included in a cowboy film "The Stranger from Ponca City" (1947). He formed the Lone Star Cowboy Band in 1934 and had appeared in 11 Hollywood films. I surmised that Lewis's long musical history might have made him the earliest candiddate to attach a musical twist to the word but this Bob Miller reference seems to predate even Texas Jim's.