OK, I owe you one—coffee, maybe? 
That worked perfectly! I had no idea I could do that. I just shared a Markdown document from DTTG to iA Writer, made a few edits, and closed it—and the changes showed up right in DTTG. That’s fantastic!
With the new Working with Documents features, I could actually see myself moving not only all my note-taking to DT/DTTG, but possibly even my blog writing as well.
Thanks so much for the suggestion!