Can you explain a little bit more why you would like to to this? Because the current direction will not work. $currentUser is filled with a guid of the user, which is a Long value to uniquely identify an Object. Where your lastname is probebly an alphanumeric string.
If you would like to retrieve any object that has an association to the User entity you should use something like (Assuming you have an assoiation to System.User)
[ Module1.Object1_System.User = '[%CurrentUser%]']