Hi Kris,
Right now, save() ends up running the fr-create-update-attachment-submission submission in universal-submission.xml, which does a replace="none", so whatever is returned by your persistence API during a save() gets discarded.
Instead, would your JavaScript be able to make an Ajax query to a service you provide after the save(), and depending on the result take the appropriate action? I think this would reduce the coupling between the Orbeon persistence API and what you're trying to do (i.e. not trying to add this information "on top" of the persistence API, which is a way is a chance of that API).
Alex
--
Follow Orbeon on Twitter: @orbeon
Follow me on Twitter: @avernet