This is probably an off the wall suggestion, but would there be any potential of capturing the referring domain in a multi-site install?

Here is the issue I face:

I am setting up a multi-site install for 26 different web sites. We are separating the database so that each site has it's own articles and information, but they all share a common database for user authentication, so that a person who subscribes to one site has subscription based access to all sites.

Using Views I can pull a list of subscribers, that part is easy. But there is no way to pull a list of subscribers to an individual site since they all share a user database.

Occassionally (more often than I would like) PayPal does not report a cancelled subscription back to our Drupal website. The subscriber will stop paying, and retain subscription access. I have to go in at the end of every month and compare the cancelled subscriptions to our subscriber list to make sure they don't still have access.

That works well for a single site. You pull a list of every subscriber through Views and compare to make sure people who cancelled don't have access to premium content. But once I hook all 26 sites into one central user database, there will be no way for an administrator at each site to pull a list of just their users to compare against.

If there was a way to capture and display the referring domain, it would be a lot easier for a person working on Site A to scrub that list, and Site B to scrub their own list as well.