The current version of WebKit being used is r12177 - you can read a bit more about the state of the sp by looking at the release notes
here. At the moment the sp download is password protected but the impression given by Ken is that either the next release or one soon after that is going to be made public (though that, of course is going to be up to OmniGroup... don't take that as gospel).
Btw, the link you provide works just the same in OmniWeb 5.5 sp3 as it does in Webkit r12177.