I can't speak for Leopard-WebKit directly, but I have used TenFourKit, which is currently at WebKit 533.21.1, which is equivalent to Safari 5.0.5 from April 2011. I use it with the installer scripts [that Tobias graciously provided at my request] to install the updated WebKit as a replacement to the system wide framework. By doing this I can use the updated WebKit with anything that relies on the system framework, namely stock Safari and most other WebKit browsers with the notable exception of Omniweb. I normally use it with Sunrise 1.8.5 for a nice blend of lightweight front end and new[er] back end.

I do not know if you can do this with Leopard-WebKit, but it is currently at WebKit 537.73.13, which is on par with Safari 7.