Is it possible to use mac trackpad gestures on, say, a linux virtualbox machine? Specifically, I would just love to use the trackpad to do "three finger swipes" and switch between Linux workspaces (desktops), that would be extremely slick.

I imagine this might require a plugin or some such thing, in addition to the disabling of the underlying Mac OS's responding to the gestures, which is what it currently does.
Using BetterTouchTool on the Mac host machine,use a application specific rule for the VirtualBox VM to map the swipe gesture to keyboard shortcuts to change desktops. To tie the rule to the VM and not the prefs app, issue ln -s /Applications/VirtualBox.app/Contents/Resources/VirtualBoxVM.app ~/Desktop/VBVM.app, to create an alias to the VM's app bundle on the desktop. Next, make the rule and set the app as the alias on the desktop. Finally delete the alias on the desktop and try it out.