Wouldn't the simple solutiion be to not start an xbiff from your .Xsession manually, but use the panel session management features to start one and just one on the panel of your choice?

I don't start one from my .Xsession manually! The only xbiff I have ever deliberately configured is the one the GNOME panel starts up and immediately swallows (which I configured using the "Add to panel" right-button menu).

The spare ones appear in .gnome/session, apparently out of thin air and in spite of all my attempts to tell them very loudly not to.