Based on the replies received, it does sound like at present that if cross-posting is a consideration, it is something done locally, and not something that is explicitly declared when federating outward or retrieved via AP.
There are also multiple definitions of cross-posting:
Multiple, disparate topics sharing the same attachment url (PieFed)
Author targeting multiple audiences
Non-author sharing object to additional audiences
One of those is really not like the other, which does complicate things somewhat. Thankfully, it does seem like that the way PieFed handles it, is local to the instance.
A good first step might be to narrow down the definition of cross-posting—at least from a protocol level—to a combination of the latter two:
> "A user (which may or may not be the object author) sharing an object to additional audiences"
Of course, this also happens to be what I'm looking for: the association of an as:announce activity with an as:target pointing to an as:Group actor.
Would this be of interest to anybody here? The fallback mechanism is to just treat the announce as usual.