Thanks for the response Steve, unfortunately it's not the solution.
We're already replicating software inventory up, and that works fine. Unfortunately, we want to populate the software library-installed software list on the parent so that when you add a package to the library on the parent, you can match up to an existing one.
I logged a ticket and was advised that this scenario is not supported, fullstop end of story. Very disappointing because I don't know why you wouldn't want to do things this way.
It means that if a customer has a hierarchic environment, they cannot go to the software library\installed software list on the parent and actually see what's installed across their fleet of computers. They have to use the old standard inventory reports instead.
Basically, when you go to the child NS's software library\installed software list, you get about 20 thousand entries. On the parent, you only get about 200 or so, because it's only listing the software installed on the NS and its site server.
Perhaps a useful inclusion for 7.1? I did ask the support guys to tag it as a feature request but I don't know if it went anywhere.