The Long Forgotten blog — my best source for scholarly discussion of the Disney Haunted Mansion and spook houses more generally — tackles the historical origins of the rides' haunted organ and the ghostly hitchhikers. It's a timely piece, as I published the long-mothballed comic that Christopher and I made in 2007 to explain the origin of the ghosts in the organist's pipes.
"The Canonic Curse" is a better than average supernatural thriller about a demonic, medieval musical composition that has a rather nasty effect on anyone who plays it three times. You can read the whole thing HERE. However, there's nothing in the story that looks to me like a direct inspiration for the Haunted Mansion's ghost-infested organ. For one thing, it's not the organ but the musical score that's demonic. For another, no visible ghostly forms emerge from either the sheet music or the organ in the story itself. If there's a HM inspiration, it's more likely coming from the illustration above rather than from the actual tale. The sketch shows a ghostly figure emerging from the musical text, but without looking closely the figure could easily be read as coming from the organ. (Frankly, it's not a great drawing.) And the caption reads, "From the smaller organ raved up a pandemonium of…ghoulish execrations." (There are two organs in the room.) In the story, the "ghoulish execrations" are sinister presences in the form of sound, but the illustrator has to draw something to represent that.
Whether or not Marc Davis or one of the others saw this sketch, it is the only depiction I have seen of an organ spewing out spirits as it is played.
But hold on a sec. "The Canonic Curse" sends me back to the ballroom for a fresh look. For some stupid reason, it never occurred to me to think of the musical composition as the thing that actually summons the banshees up and out of the pipes. The tune grows more frenzied as it sails along and disintegrates into a mad improvisation as it reaches its climax. It's as if the music were the thing compelling them to appear.