Here is my take on putting the final field switch into the policy file:
https://github.com/stephenh/scalagwt-gwt/commit/35535b2f36cb196792a411430567a7a9aeb8adb1 Now final fields only go across the wire if the user opts in, and the server-side behaves accordingly. http://gwt-code-reviews.appspot.com/1380807/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
