At the moment torn-off menus will not reflect changes made to the
original menu.  This functionality is slated for an upcoming revision of
the `menu` system in Tk.  It`s possible to do this now by tracking the
associated tearoff using the `-tearoffcommand` option for menus.  This
allows you to specify a command that will be executed when the menu is
torn off.  The widget names of the menu and tearoff menu (tearoffs are
created as toplevels with the name `.toplevel.tearoff` when torn
off) are appended to this command before it is evaluated.