While porting Evolution to WebKit2 I hit strange bug. The callbacks from webkit_web_context_register_uri_scheme are not called when the request with given URI scheme is called. Actually it's working fine in webkitgtk-2.0.4, but not working in webkitgtk-2.2.2 and in trunk. You can test the code from wip-webkit2 branch in Evolution repo.
hmm, unit tests pass and about: handlers in ephy work here too.
Closing as it was caused by misuse of the webkitgtk APIs.
Is it true that 'about' is a reversed scheme, and the right use of the
APIs is using something like 'xxx-about' and use tricks like
If so, it's better to document this in the reference manual.