This is actually not a bad idea, like a little red exclamation mark or no-wifi symbol or some other clear visual indicator next to a non-federating ActivityPub user's handle. And/or the symbol could show up on one's own replies if there are any issues present, maybe with a hover-over pop-up that lists who in the thread can and can't see your reply.
I'm not sure how difficult this would be to implement technically, but something in the UI to help the user understand federation / bridging issues would definitely be helpful, be it for issues caused by defederation, or for issues on our end with sending out the posts.
