Second Save Click for Anonymous Users

When a user without Update permissions (in my case an anonymous user) clicks the Save button a second time they receive a somewhat scary message about a database error. Scary because they may have just spent a significant amount of time filling in my form and don't want to lose data. #1619 suggests that the Save button should be hidden. I don't know how large of an effort that would require. So I'm looking for alternative solutions. I had the idea that if I added an additional control to my form I could use that control to indicate that the form had already been saved. 

Is there any xpath expression to use in the conditional that would not require adding an additional control? 

    <property as="xs:string"  name="oxf.fr.detail.process.save-final.MyApp.MyForm">
        require-uploads
        then validate-all
        then (
          save
          then (
            xf:setvalue(ref = "//form-saved", value = "'true'")
            then success-message("save-success")
          )
          recover if ("//form-saved/text()='true'")
                    then error-message(message = "This form has already been saved. Further modifications are not allowed.")
                    else error-message("database-error")
        )
    </property>

The PDF button can still be invoked after save. If a user forgets to make a PDF for their own records before save they can after. But maybe it is possible to send to the PDF? I'm afraid the process/button docs are still mostly a mystery to me. Is this possible? How? And I would still like to know if there is something testable with xpath that indicates a form has been saved.