This is an interesting additional complexity.  Usually Buildbot's fix for this sort of thing is to allow a callable to be passed in the configuration.  In this case, that callable would probably take a SourceStamp or a BuildStatus, but unfortunately both of those classes will be removed in Buildbot-0.9.0, so it would be a shame to introduce a new API depending on them.  Perhaps the status plugin can take a alwaysUseGithubRepo="user/repo" directive to force the given repo even if the repository in the sourcestamp does not?