Hi Robin
My first reponse was based on retrieving the data you were after from within an Orbeon application. If you want the data from an external request, it's slightly more complex.
/fr/service/persistence is relative to the Orbeon root, so an external request will need to include the Orbeon context eg.
http://localhost:8080/orbeon/fr/service/persistenceOrbeon Services, however, come with default protections for external requests (see
http://wiki.orbeon.com/forms/doc/user-guide/form-runner-user-guide#TOC-Securing-Form-Runner). I'm out of my comfort zone on talking about that, all my stuff is internal.
Alternatively, you could go direct to the 'actual' persistence layer. If you are using the embedded exist then you can POST xquery directly to the eXist REST interface eg.
http://localhost:8080/orbeon/exist/db/orbeon/fr/app-name/form-name. Again, this is protected by default but can be opened by modifying your web.xml (note disclaimer at
http://wiki.orbeon.com/forms/doc/developer-guide/release-notes/40#TOC-Security-of-the-built-in-eXist-database).
Or (preferred) create your own standalone eXist database protected with your own security model, and modify your Orbeon properties-local.xml with the oxf.fr.persistence.exist.exist-uri property pointing to the standalone eXist REST interface.
Regards
Jez