I don't know anything about castor other then there is a castor.jar in the 3.0rc1 lib directory. Is that the jar your are looking for?
No, I was looking for the one listed on the JBossCastor projects page. The link does not work there anymore.
... which is only working with JBoss2.4.4
You have to wrap org\jboss\util\ServiceMBean and org\jboss\util\ServiceMBeanSupport to work with JBoss3.0.
Both classes are now in org\jboss\system\