Major Description
As proposed on dev [1] I have hacked together a small piece of code which implements support for the whiteboard pattern for registering MBeans with MBean servers.
The code is in my Sling whiteboard at http://svn.apache.org/repos/asf/sling/whiteboard/fmeschbe/jmx-whiteboard but is intended to be added to the Aries JMX functionality.
The single-class implementation picks up DynamicMBean servies registered as "javax.management.DynamicMBean". Each such service is registered with each registered MBeanServer service (similar to how the framework MBeans are registered by jmx.core). The JMX ObjectName can either be provided with a "jmx.objectname" service registration property (String type) or the DynamicMBean may implement the MBeanRegistration interface in which case the ObjectName is provided by the preRegister method during registration.
This is just a first hack which has not really been tested but mainly serves to show the idea. It is currently implemented as a Declarative Services component but may as well be implemented as a regular class using ServerTracker instances and instantiated by the BundleActivator (which may be a good idea to make this as widely usable as possible - for example in Sling we don't currently use Blueprint, so a Blueprint based implementation would be a no-go for us 
)
If there would be interest in this, I would try to test and validate this piece with the hopes to get a release soon (may be released independently, hint hint )