I am getting a 500 : Internal error, the payload of the POST is definately right, one thing I noticed is the Content-Type is not "sticking" using b.addheaders = [('Content-Type',"text-x-gwt-rpc")] -- and I'm not sure why.
Does anyone have quick/dirty code used to access a GWT RPC (i.e. emulate the GWT client) in Python using mechanize.  Browser?
Do not use GWT-RPC from non-GWT solutions, the format is internal and subject to change without notice. Consider RESTifying the app (or use SOAP and reuse the existing code of the methods, but care needs to be given to the interface definitions).
Calling a GWT RPC function from Python-mechanize