One approach would be to write an on-startup applet which removed the rules menu item using the GroupWise.RemoveMenuItem() method.
You could then use the Out of the Office rule applet, configured on a toolbar or menu, to allow the user to create the appropriate rule(s).
You would also have to consider providing a mean by which users could remove the rules when they were no longer required. I suspect this would require another applet designed just for this task.
I hope this helps.