/Developer/Examples/WebKit/NetscapeMoviePlugIn/
One thing I would not recommend is using any of the QuickTime API calls in that plugin, they are laughably out of date. Nor was it obvious, to me, how you add things like JavaScript properties. But it is a start for an XCode project.
And I won't get started on what a sorry state of affairs that something so archaic as Netscape plugins are still used to add functionality to Mac browsers.