JBoss uses org.jboss.mx.util.MBeanProxyExt itself. For the average JMX user, javax.management.MBeanServerInvocationHandler seems like the correct interface to use and sufficiently good.
Is there some reason to use one over the other, besides the additional features of MBeanProxyExt? Is one legacy?
These docs seem to use it:
http://docs.jboss.com/jbcache/1.2.4/TreeCache/html/
I believe MBeanProxyExt is legacy, before MBeanServerInvocationHandler was standardized. It had some equality problem, too:
http://jira.jboss.com/jira/browse/JBJMX-2