Nearly there with the rss -> activitypub script first cut, but slightly hampered by the fact that it seems like there's a bug (github.com/bonfire-networks/...) in the handling of Update activities in Bonfire's client to server #activitypub API. Either that or I'm holding it wrong, which is entirely possible given it is the first time I'm using the API!
As an aside, I'm quite liking the ActivityPub APIs so far, although ld-json does leave me feeling like there's a fair jump from "script that works" to "actually does everything the spec would theoretically allow".
Hmm. That said, I also seem to have lost the ability to update posts via the UI on #bonfire 1.0.2. I didn't spot exactly when the option disappeared from the menu.