Which is to say, all Shimano rear deraillers designed for indexed shifting have the same cable-pull ratio. (Excepting pre-9-speed Dura-Ace, which probably isn't your worry.) So you could use a 105 rear derailler originally from a 6-speed gruppo, with 9-speed cassette and shifters. I've done this and it works fine. You could also go the other direction.
This isn't nearly so clean with Campagnolo, fyi.