Short answer: I couldn't figure this out, but I found a different way to accomplish my goal. Details below.
My goal was to have a user fill out a list entry, and saving it would trigger the creation of a document set containing a 'receipt' document holding all the information the user provided in the list entry.
It sounds simple enough, but there are many gotchas:
Can't create a document in a SharePoint list, a Drop Off Library, or a document set via SPD workflow.
Can't use a custom InfoPath form as a front-end for document library submissions.
Can't Copy files to a document set via SPD workflow.
Can't use variables for target paths in Content Organizer rules, so dynamic rules (like those routing to newly created document sets) are tricky.
Can't directly create documents in the Drop Off Library via SPD workflow. (The original goal of this post.)
For these reasons and more, my solution has four pieces: a list, a 'holding' document library, a Drop Off Library, and a final document library for document sets.
The list has an InfoPath front-end. Users access it and fill out the InfoPath form to create a list entry. When the entry is created, a workflow kicks off to create a document set in the final document library, update the relevent Content Organizer rule to route content to it, and create a 'receipt' document in the 'holding' document library. The creation of the 'receipt' document kicks off a second workflow that makes use of an action not available in list libraries: "Send Document to Repository". This action is IMPORTANT because sending files to the Drop Off Library this way actually triggers the "Submit" event that routes documents around SharePoint; simply Copying files to the Drop Off Library will not!
The workflow action looks like this:
Submit File using **this action** to **this destination router** with **this explanation** (output to **Variable: submit file result**)
- this action: Select "Copy" or "Move". I went with "Copy" so the 'holding' document library can serve as an easily auditable store of 'receipt' documents, not just an empty document library that looks unnecessary to anyone unfamiliar with this ridiculous Rube Goldberg setup
- this destination router: Type in JUST THE NAME OF YOUR SITE (i.e., https://sharepoint.com/branch/site/). Several guides mention '/_vti_bin/OfficialFile.asmx' or whatever, that never worked for me. Similarly, putting the location of the Drop Off Library also didn't work. The site URL worked, despite all logic and reason saying it shouldn't.
- this explanation: the workflow status you want logged. I just wrote "Completed".
- submit file output: unless you have a strict variable naming scheme, just leave as is. Harmless.
This action copies the newly created 'receipt' document to your site's Drop Off Library. The Content Organizer rules, having been modified in the first workflow to point to the new created document set, will correctly route your 'receipt' document to its final location.
Terribly complicated, but it gets the job done in a non-coded fashion. I hope this helps someone.