I actually came here for the same question. I was going to hack it... but I wasn't sure that it would work correctly with 10.5.7. I got used to the 3 and 4 finger gestures and now I want them back! I wonder why Apple doesn't just officially support it instead of us having to hack it. They don't loose anything by supporting it, do they!
If someone with TimeMachine can try it to see if it works safely, we'd appreciate the input!