There are system-wide mouse gesture solutions from 3rd party vendors you might want to look into, here are the ones I know of:
FlyGesture
http://www.flyingmeat.com/flygesture/
This one is free and actually the most Mac-like. I don't know whether it's enough functionality for you.
xGestures
http://alum.hampshire.edu/~bjk02/xGestures/
This one should work for all applications
CocoaSuite
http://www.cocoasuite.com/
THis one works only with Cocoa Applications (OminWeb is one), so some older applications or cross-platform applications might not work with it.