Is it possible to reassign the "Pinch with thumb and three fingers" gesture to activate Dashboard instead of Launchpad?
I'm not sure if there's a plist modification you can do to alter the behaviour, but I do know that BetterTouchTool will let you replace the pinch zoom in/out gestures with custom actions. You can assign them to activate Dashboard instead of LaunchPad if you like or any number of other things. It can be a bit involved to set up, but it is very powerful in what it lets you override from the OS X defaults.

TUAW has an article on customizing a Magic TrackPad using Better Touch Tool that demystifies it a bit.