I guess the explanation is pretty clear, in Scott's comment on that old review. BSM was only concerned about api changes, in terms of whether other java source code would need to be recompiled, that depended on it, etc.....
RPC and ClientBundle are indeed concerned with internal fields and annotations of classes, so an extension to BSM would be needed. http://gwt-code-reviews.appspot.com/1446818/ -- http://groups.google.com/group/Google-Web-Toolkit-Contributors
