I am headdesking all over the place here. What I have learned, again: don't assume things are complicated, try the simple stuff first.
To wit: if you want an RSS feed of something to show up in your network, go to your Manage Connections
page, put the feed URL (e.g., "http://feeds.feedburner.com/Metafilter")
into the "Add Connection" box, and click Add.
(I think) create a channel that connects to an RSS feed and also republishes it (which is what I was trying to do originally, without realizing that you literally need to add the connection for it to work), which other people can then connect to, but it's completely unnecessary.
This is stupidly easy, you guys. Want to add an AO3 feed for your favorite pairing? Go to AO3, filter to the pairing you want, and right-click-copy on the RSS button at the top of the page, then paste that into an "Add Connection" box. Boom. This is what to add if you want all Person of Interest Harold Finch/John Reese fanworks, for instance: https://archiveofourown.org/tags/287761/feed.atom
(And because it's a regular connection, you can add Custom Filters to white- or blacklist any terms you don't want. Because of course you can be that specific!)
ETA: Dreamwidth and Tumblr both automatically provide RSS feeds of any user.DW: https://accountname.dreamwidth.org/data/rssTumblr: https://blogname.tumblr.com/rssTumblr but embedded in a custom domain: https://customdomain.xxx/rss
So basically you can replicate your flist/dwircle/dash on Hubzilla, all in one place. Including subbing to or excluding tags, because of the custom filtering.
THIS IS SO EASY