Hi Alec and all PKP Team,
First of all, glad to see the support team growing!!
We are evaluating a Zopen Archives Harvester product for Zope/Plone, which categorizes the content.
We are not sure how it works yet, because it won't install in newer versions of the CMS.
Here's a demo of the search, which apparently is run on the XML collected.
What we are assuming is it might use the ListSets to categorize content, but it may be implemented differently.
The idea behind categorizing content is to separate the types of repositories harvested: journals, conferences, digital libraries, institutional repositories, whatever classification people may want to come up with.
So, when viewing the list of repositories, you could have something like
this example, which is a modification made in DSpace, with CSS+Javascript, when listing categories. If you click the "»" symbol, you'll open the list (useful when hierarchy is deep!).