Actually it's not necessary to implement a new message interpolator. Using HV's ResourceBundleMessageInterpolator and its bundle locator API a custom bundle name can be specified like this:
Code:
Validator validator = 
   Validation.byProvider(HibernateValidator.class)
      .configure()
      .messageInterpolator(
         new ResourceBundleMessageInterpolator(
            new PlatformResourceBundleLocator( "my_bundle" )))
      .buildValidatorFactory()
      .getValidator();
This doen't work using the XML configuration at the moment, though. I generally like the property approach, but I'm slightly concerned about possible inconsistent configurations (specifying the property but using a different message interpolator etc.).
--Gunnar