I know this is an old thread but i just came across the same issue. I resolved it by making my DLQ NOT a gateway.
So i have a queue defined called retrieve_search_certificate_esb. it has a DLQ configured as such:
<attribute name="DLQ">jboss.esb.destination:service=Queue,name=retrieve_search_certificate_dlq</attribute>
which is in turn mapped to a jms-bus with a busId of retrSearchCertDlqChannel
that jms-bus is configured by my service to be listened to by my service with
<jms-listener name="ESB-Channel" busidref="retrSearchCertDlqChannel" is-gateway="false"/>
the part that gave me access to my org.jboss.soa.esb.message.Message is: is-gateway="false" When is-gateway was true, the ESB Message was serialized to a String in the same way as the original post in this thread.