Yes, you can make XMLHTTPRequest backend calls, although it's a bit of a pain to debug due to same-origin policy. If you get to this basically you have to turn off built-in GWT server and use your own to serve both GWT-compiled code and your XML requests.
Doesn't require some third-party (which tend to lag behind the latest versions) library to get it working and I can write normal server side code in Python, etc.
I've done the same in the past (Python backend). I'll add one more suggestion on top of that. You can create JSNI bindings for all of the JSON RPC objects with GWT to get compile-time API checks that disappear entirely from the output.