SOLVED It took a while but I got around to doing this. It was pretty simple, I was just confused about the different options and how they were different from each other.
Short version: create a content file with a layout parameter in front matter. The value of that parameter should match the name of a layout file under layouts/_default/.
Real example Git commit where I got this working: https://github.com/xaprb/story/commit/6e31bfaf5afbabfcb184447454d7e2801f67fd9c This produces a special-purpose RSS feed at the URL /mailchimp/index.xml