setHarURL was needed simply to allow the deployer to inject this value into the mbean. Use of this at runtime or through the console is highly discouraged ;)
Another thought is that in the scheme y'all are proposing (provided multiple interceptors can process the deployer init stuff) we could simply have an additional Hibernate-specific interceptor which checks the deployment's mbeans for the Hibernate mbean and injects the url. Then you would not need ServiceMBeanSupport exposing the URL attribute at all and could still get away with no specific HARDeployer.
Correct, and this is certainly the future deployment process architecture as a deployment is just a packaging with instructions on how to configuration the various aspects.