I am having the same need for this. Initially I wanted to suggest that all /ValidationMessages[_*].properties shall be loaded from the classpath but the approach suggested here is better as it prevents problems with classpath ordering.

Please also note that GWT provided their own way to address this problem. See at the end of this:
http://gglwebtoolkit.blogspot.de/2011/12/client-side-bean-validation-with-gwt.html