The fans at Atlanta Braves games have been saying "BraveS" with emphasis on the plural 'S' for many, many years. I'm sure the idea existed even before that, but it would make sense that it started with the Braves, being so close to the original word in the song, and then expanded from there. Who knows. Like many things, it probably didn't have just ONE original source, but was a concept that made sense to many individuals and groups around the same time and expanded from many places.