Think out loud...
I think this spec need not be overly limited to LDP containment per se even in the context of Solid.
Express things in a way where it'd be compatible with both LDP Containment as well as ActivityStream Collections.
In other words, while this spec can be "Solid-PREP" but it'll still be compatible in an ActivityPub environment, or even Solid environment that's including an extension using profiles: application/ld+json; profile="https://www.w3.org/ns/activitystreams.