Thanks for the feedback 
I gave this some more in-depth thought yesterday as the topic was bumped (which is why I ended up posting here). I think a client-only solution (i.e. a javascript embed) is the only one that really makes sense here. Otherwise we’re essentially talking about a number of platform-specific implementations, each with their own set of issues.
You’re right that concurrency and load are issues. There are significant load and concurrency issues with ActivityPub as a single ActivityPub post can open you up to many incoming and outgoing requests to and from the Fediverse. In that context, this may be actually slightly easier as the remote client(s) are controlled. Moreover, in this case, concurrency and load are only really issues on the server side (i.e. the Discourse side), and, while they are issues, I think they should be solvable via background jobs, caching and mutexes. But yes, important issues to consider.