You're basically trading a compile-time error for a runtime error. I must say I don't quite like it.
IIRC, the Editor framework mandates that the editor type (from the Editor<T> parameterization) and the edited type (from the property or field, or the EditorDriver parameterization) must be the same. See http://code.google.com/p/google-web-toolkit/issues/detail?id=6016 and the linked discussion from GWT-Contrib. http://gwt-code-reviews.appspot.com/1860804/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
