You should be able to do this from the MBean. MBean just manages whatever resource that you need to manage. IF you want to manage a EJB, do the lookup for that EJB in ur MBean create/start methods and expose the other methods that you want to manage. These methods can inturn call the actual EJB Resource and do whatever.
IF you just want to time the methods, JBoss provides a EJB Interceptor called LogInterceptor that you could just add to your interceptor stack. That will automatically determine the time taken to run every method. Check out standardjboss.xml for LogInterceptor.