I understand that you can:
// Where CustomMacOSXController implements implements MRJAboutHandler, ...
CustomMacOSXController macOSXController = new CustomMacOSXController()
MRJApplicationUtils.registerAboutHandler(macOSXController);
MRJApplicationUtils.registerPrefsHandler(macOSXController);
//... and so on
However I can't find anything to hook the copy and paste menu so that when it's called, the currently selected JTextField will be affected (for example the paste will paste the clipboard into the selected JTextField.