I tried calling this method as follows:
    mobility = MobilityHelper ()
    mobility.SetPositionAllocator ('ns3::RandomRectanglePositionAllocator',
                                   n1 = 'X', v1 = RandomVariableValue (UniformVariable (-5.0, 5.0)),
                                   n2 = 'Y', v2 = RandomVariableValue (UniformVariable (-5.0, 5.0)))

But I get this error:
mathieu@ns-test:~/code/ns-3-sigcomm$ python ./examples/sigcomm.py 
Traceback (most recent call last):
  File "./examples/sigcomm.py", line 168, in <module>
    main(sys.argv)
  File "./examples/sigcomm.py", line 75, in main
    n2 = 'Y', v2 = RandomVariableValue (UniformVariable (-5.0, 5.0)))
TypeError: argument 1 must be ns3.PositionAllocator, not str

which shows that the python bindings seem to expect me to call the overloaded version of that method which takes a Ptr<PositionAllocator> argument.