That’s a good idea!
Create a topic somewhere hidden; make it unlisted; move it somewhere public. Add .json to the copy URL to get the post_id of the OP. Then use that URL in the permalink. Then you can get the URL of the raw text of that post with a URL like /posts/1093/raw (where 1093 is the post_id of your topic.
Then https://dashboard.literatecomputing.com/.well-known/security.txt will get you a text URL like (I think) you want. Much easier than mucking with having app.yml muck with nginx settings.