@silverpill I looked at this (and made a tentative internal proposal to adopt this) for the Discourse Activity Pub plugin. We didn’t include anything along these lines in Phase 2 of work on that plugin, however it may be possible at some point.
I should be clear that CDCK (Discourse.org) hasn’t endorsed any version of this for their plugin, the Discourse Activity Pub Plugin, yet, however in my own, personal, view (as the current developer of the plugin), I think adding support may be feasible down the line a bit.
Where can I see your implementation? Is there any reference implementation in Ruby? 
Also, is anyone here familiar with the folks involved in Mastodon and/or have a real (i.e. not just speculation) sense of the likelihood of adoption of the same in Mastodon? Mastodon adoption, at least in principle, would obviously be an advantage in advocating for adoption of this in Discourse’s plugin.