@evan yeah, imho jwz had the right idea: replies to a post should be a timeline/collection, curated by the author, hosted with the the post...
"inReplyTo" already allows thread-chaining for clients that want to assemble a thread view, AP already has excellent timeline/collection support, all it needs is a field on the post (like "replyCollection"?) to indicate the id of this collection.
this could be a short FEP. i'd just want to try implementing it in my server first to make sure it'd work.