Crossing my fingers, I think I finally got it to work reliably, with a Close Form rule. I had to add a simple workflow to the first iteration of the form that populates the URL path attribute. I used SharePoint Designer to construct the workflow. It has 1 step, if the current item of the URL variable is empty, the set the variable to the encoded absolute URL of the current item. This runs on the target library when new items are created.
Then I added a rule when a button is clicked to save the form to the next library. Here's the qRules command: concat("DeleteFromSharePoint /url=", @qRulesURL)
I've tested several forms in a row, making sure the ID of the form doesn't contain special characters. That was another issue I ran into, but thanks to a previous lab by Hilary I used the translate() function to replace those characters.
Can anybody see any holes in this approach, or have any idea why the command won't work without the workflow? My theory is the actual encoded URL points to a different path, since our outward facing pages are secured in SharePoint by application certificates, not the entire site.