I think the JBossMQReceiverImpl code will explain how the load balancing works. I haven't read that. I verified that when the messages are queued at a rate higher than Master node can handle, the other nodes in the cluster start receiving the messages.
cdot.jboss.JBossMQProvider class is applicable to version 3.2.x only. It provides a way to look up the Queue in a clustered environment using comma separated list of JNDI URLs. It doesn't have anything to do with the # of MDBs and how they are assigned.
In the cdot-jbossx.jar, If you see how the ProviderUrl is defined in jms-ds.xml and the implementation of cdot/jboss/JBossMQProvider.java, you will see its role.
JBoss 4.0.x already took care of this. So, even if you take the cdot-jbossx.jar, everything will work fine.