Ah, the dangers of reflection.
In the course of working on JBREM-727 ("Make Client's implicitly created Connectors accessible") I changed the field Client.callbackConnectors to map from an InvokerCallbackHandler to the *set* of all the Connectors with which it is registered. Since Messaging's JMSRemotingConnection always creates a new InvokerCallbackHandler (CallbackManager, in particular) for each connection, callbackConnectors should map each CallbackManager to a singleton set. I.e.,
Connector connector = (Connector)callbackConnectors.get(callbackHandler);
Connector connector = (Connector)callbackConnectors.get(callbackHandler).iterator().next();
Thanks... it fixed the testcase.