(Disclaimer: Not having the specs up at the moment so I'm going off the top of my head)
The java:comp/env namespace is only usable inside EJB's. In there you find "logical" names for other EJB's (or other resources), that you have configured in the deployment descriptor for the bean doing the lookup.
It might be that the beans are not available yet anyway during MBeans startup, but if they are you should succeed in finding them by using the "real" JNDI name for the bean you are trying to access. (The Beanname in ejb-jar.xml or the jndi-name set in jboss.xml)