Conversation

it feels like an unnecessary abstraction for the purposes of skirting around a limitation in the ActivityPub specification.

What limitation?

The problem is not that ActivityPub has a limitation, the problem is that it doesn't have enough. It can't be used to build a real application because it doesn't specify what is valid and what is not. it doesn't even specify what an "actor" is.

Fortunately, the answers to these questions were found and documented in FEP-fe34 and FEP-2277. Object observers are likely compatible with both FEP-fe34 and FEP-2277. Other ideas are not compatible.

In your proposed structure (feel free to correct if wrong), a resolvable context would declare an observer property pointing to an Actor, who would be federating actions out on its behalf.

Yes. I think some property can also be added to posts to simplify discovery e.g. Note.contextObserver.

However, it has the same technical hurdle — lack of existing implementation — than the alternative, which is to multi-type the collection into ["OrderedCollection", "Service"] or similar.

So ["OrderedCollection", "Service"] is supposed to be an actor that is also a dynamic container? That doesn't make any sense, and I don't know to implement that in C2S setting. It also conflicts with FEP-fe34 and FEP-2277.