So the spec says that replies and such should be forwarded, which helps with thread completion. This is only implemented by a few projects, and relies on LD signatures, which others are not excited about.
What I’ve done in honk is to send a Read activity with the reply as object. Not many other implementations support this, but for those that do, it’s a hint that another object exists and is part of this thread. Then they can fetch it from the origin if needed. I’m currently repurposing Read activity since it seems relevant (here’s a reply, I read it), but maybe somebody wants to use it as a more limited read receipt type thing.
However, I think the idea is fairly sound, and what would help is a Notify activity to tell other servers about objects and activities they may have missed.