I have made a complex models structure in django. I had planned to render this structure using GWT, but with python the option to communicate whit it are: - using JSON and
I've actually crossed this exact bridge, and unfortunately I found it is best to make the client side objects in GWT. Luckily, however, this is easily done using JavaScript Overlays and a little code generator.
My code generator looks at the models.py file and rips out all of the pertinent fields before plopping them into the .java files for GWT. Once you have this, implementing your new objects is pretty easy.