I don't really have a use for Launchpad, but the gesture to me makes sense for Dashboard. I experimented a little with BetterTouchTool, but it doesn't have the gesture available, and I'm not sure it would work well with the sequence of successive pinches and spreads to go from desktop Expose to overlay (normally Launchpad).
Ideally I'd like to see if there's a plist tweak that can be made somewhere that would keep the gesture behavior exactly the same, but substitute Dashboard for Launchpad.