Thanks Tom. I got this reponse from RedHat, which others may find useful too:
"These locations are no longer used by the SOA platform. The usage of these locations has been deprecated in JBoss ESB 4.3, where an attempt has been made to standardize the location of the message payload with the introduction of the MessagePayloadProxy :
'This class can be used by actions/listeners to manage access to the payload on a Message instance. The class is constructed from the configuration of the component (listener, actions etc) using the proxy instance. It checks the configuration for "get-payload-location" and "set-payload-location" configuration properties, defaulting both to the Default Body Location (Body.DEFAULT_LOCATION).'
In short, you *could* use these locations, and can expect that the stored data is not being manipulated, but the recommendation is to use the Body.DEFAULT_LOCATION instead.
Hope this helped to clarify your question. Please let us know if you have any additional comments or concerns."