Thanks Rusell. I compiled the master code base but still I couldn't find the way to link way in pb.
Have a look at this test. It is exactly the same for both HTTP and PB, it uses subclasses to create the clients, but codes to the interface.
Most of the code in the test is setup (to store the graph of linked objects), the pertinent code is on line 89. To do this over protocol buffers just create a PB client, the test creates one here.