Patrick, thanks for your insight.
I was hoping that at the moment the section gets signed, that a change event would fire. Unfortunately, I haven't found this to be the case and the rules for this seciton are useless as it pertains to a signature event.
As for the workflow, I have successfully copied values out of the XML and updated the SharePoint list item with those particular values. What I have not succeeded in is... directing those SP field values back into the form. That's a goal for another battle.
After a full day and a half of battling this beast, I found a workaround. In the submit options (under tools), instead of using the "Send form data to a single destination", I found that I could use custom action rules which check if these sections are signed, then set the appropriate date fields, and then submit. The single drawback is that the date stamp will be locked into when the form is submitted rather than the actual date of the signature (in the event that it was signed on one day, and submitted on another).
Thanks much for your help.