If you're using JMX and MX4J then you might find the Commons Modeler project useful. It contains a number of helpers for working with JMX such as utilities for registering objects with the registry, an MLet parser or helpers to register normal beans. In particular it has some useful Ant tasks for creating and registering MBeans.
In effect Commons Modeler + MX4J provide a nice BSD licenced open source JMX backbone similar to that available in JBoss (which is LGPL). Apparently the new version of Tomcat (5.0) is going to use these two libraries to provide a generic JMX backbone on which the services and plugins will run.
Many thanks to Costin for pointing me in the right direction.