Hmm, tricky one, the ObjectName in JMX 1.0 (in JBoss 3.0) is not serialization compatible with JBoss 3.2 (which is the first JBoss version that uses JMX 1.1 which was the first JMX spec to define serialization compatibility).
And I can't think of an obvious way for you to replace the 3.0 javax.management implementation since the JMX libs are part of the bootstrap (IIRC loader repository for SARs [mbeans] is only available in JBoss 3.2).
One solution (and this requires you to write some code but its easy code to write) is to create your own connector implementation which passes object names as strings rather than ObjectName instances. And then convert those strings to Object Names on the server side before invoking the MBean Server.
Connectors are really easy to write, just have a look at the current source (albeit its convoluted one) for RMIConnector or check some of the sources I included with the JMX book to get yourself forward (http://www.samspublishing.com/content/images/0672322889/downloads/code.zip).
Right now that's the best solution that comes to my mind.
Thanks Juha, I will check that out and give a feedback afterwards.
I have decided to write a thin RMI implementation wrapped in an mbean for talking to the remote instance.