Hi Richard,
The upgrade will require quite a lot of effort: recent WebKit is significantly refactored comparing to webkitdriver snapshot. Main problems in upgrade are changes in interfaces (missing methods should be implemented, usually as stubs), code location changes (class was split into two or moved into different namespace).
I think the most straightforward path would be create a patch of modified webkit against same release (55226) in webkit repository and apply this patch to very recent WebKit release. Then try to build, adding/modifying implementations of classes in hl/ subdirectories according to build error reflecting changes in WebKit interfaces.
--
Igor