This shouldn't be necessary at all -- resource lookup is resolved according to the search tree for the locale in the permutation, so when it is resolving messages for "en", it will find the ones in LocalizableResource.properties (or the annotations in the source).
Can you describe exactly what problem you are seeing? http://gwt-code-reviews.appspot.com/1337802/show -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
