If the package you're interested in is not a singleton or service, or cannot be found with site_node_closest_ancestor_package* then the proper
instance of a package is a tricky thing to define, or at least package specific, I think,...
I agree. What I thought then that I would like to see was some method in which packages and package instances can publish/make known certain attributes about themselves: the names of services they provide, perhaps just a unique id, perhaps the ids of their administrators, or their organizations, or ...? Structured in a property list, but perhaps unstructured beyond that: the name and value of the property might be left up to the package instance, and might be settable both in the package init as well as from the site map. That would enable packages to examine their where-abouts and enable administrators to bundle/config together various package instances. (Sigh, how many tenses are in this paragraph?)
