I think you are on to something here. I had imagined that singletons could be singleton per subsite, so that it was still clear its a special type of package.
This is one of the major issues I noticed with host-node mapping:
when sending notifications out, for bug-tracker for example, or generating RSS feeds, the content generates a link to the creation users profile page. This breaks if you use host node mapping. You would need to know which subsite/host node mappaing, the content was created under.
I think having acs_objects.packge_id, it is possible to scope category treess, notifications, search, etc to a subsite.
We would have to make sure folks that use subsites but don't use host node mapping were not inconvenienced. That is, subsite scoped cateogies, might not be necessary etc. Subsite search almost always should be an option.
