I don't think it is even possible to get this to work, I seem to recall trying this in the past and having some issues with it. Give it a try though:
1. Download MX4J. This is a decent open source JMX impl
2. Add this to your WAR lib directory
3. Use MBeanServerFactoryBean to configure an instance of the MBeanServer
4. Add the following to weblogic.xml:
Let me know how you go on.