Thanks for documenting your attempt! The GTK+ port of WebKit is still quite new and has some pretty significant portability limitations, so I’m not really surprised that it didn’t work.
Making a nice PyObjC front-end for Gwibber should theoretically be possible. The back-end service parts of Gwibber are entirely decoupled from the front-end, in much the same way that libpurple is separate from Pidgin. In fact, if you installed the major Python dependencies, you can probably use Gwibber’s microblog library as-is on Mac OS X.
It’s likely that certain parts of Gwibber’s front-end stack could also still be reused for a Mac OS X version, particularly the HTML themes. The hardest part of making a Cocoa front-end for Gwibber would be making a working configuration system and account manager. On Linux, Gwibber’s gconf abstraction layers made that significantly easier.