This can be easily achieved by using Tags. A user register a set of tags. Each tag is a feed channel for that user! When posting a message, the user choose a tag from a list box!
Discussion
That can also be a Community of yourself. You can select the posts you want to appear over there. Any post.
It sure works! I started using the "Community" feature yesterday (F-Droid takes time to update). I still don't know how to create a community on Amethyst 😥