I have actually suggested something along the lines of "NS:replyDisposition": "NS:Quote" or similar as a way of signalling quote posts in the past. I think there is a lot about this model which is good!
I dislike this approach because it requires indexing the URL of the reply collection in order to process efficiently. If I receive an Add with the target being an object’s reply collection, I have to identfy that the target is that special purpose collection.
I prefer a special purpose activity for this reason. We shouldn’t be scared of vocabulary extensions. In fact, I think it makes things easier to be able to just recognise the type rather than having to infer the behaviour from the URL.
If I could do a ground-up backwards incompatible redesign of AP, I would ideally un-reify things like inboxes, outboxes, reply collections, etc; which are properties of objects more than distinct objects