you are right, there's definitely space for improvement in this area. I saw you already opened BVAL-225
for this; that's great to track this.
Just one note, you might find Hibernate Validator's AggregateResourceBundleLocator
useful. This allows to merge several resource bundles into one. So if you have the constraint modules under your control, you could put a uniquely named resource bundle into each module and create an aggregated bundle locator for all these bundles. That way each constraint module would come with its own resource bundle while the texts still can be overridden with the ones from ValidationMessages.