The name of the song is, "The Second Arrangement." I know a guy with a bootleg rough mix. It's a decent tune, but Gaucho didn't suffer from it not being there. It's honestly not that great a song; my sources tell me that Becker and Fagen just didn't want the hassle; a few albums earlier, Fagen had tossed a synth out the window because of the amount of hassle it gave him. I guess they weren't big on patience at that point of their careers.
Becker and Fagen, to this day, deny the existence of the track, but I've heard it, and there's no mistaking that voice, or the song's style, anywhere. We thought about recording our own version and releasing it, but I'm willing to bet it's copyrighted, despite the Steely Dan's protests that it doesn't exist.
EDIT: One of the members of the band in question is an acquaintance, professionally, with Walter Becker, but more importantly, Roger Nichols. So I've heard a few details of the story; the studio intern was supposed to make a copy of the mixdown to listen for mastering purposes. He copied the blank tape over the original, rather than the other way 'round. I don't know for sure, but I'm willing to bet he hasn't worked much since then.