Sorry but we haven't made any progress on my scenario, we're still using a patch that I created for Remoting 2.2.3SP3 that applies a timeout on the callback connection. A couple times a day all the message activity comes to a halt and then it recovers after the timeout is reached. Back then when I tried to upgrade to 2.2.4 we got a different issue and we didn't pursue it farther because we we're comfortable experimenting with new JBoss or JBM releases.
Your getMessageCount block seems a little different from what I was seeing. I wouldn't think that is related to the callback handlers. I wasn't using XAConnectionFactory in my clients because we have ways to resend the messages if we had to. I assume the XA adds complexity and opportunities for blocking.