You should be able to build a report from the WSUS Reports -> Windows Server Update Services folder to accomplish this kind of list (all currently published 3rd party updates).
Go to WSUS Reports -> Windows Server Update Services and create a New Report under that folder.
You'll want to add the Update Title field from the Updates datasource.
You'll want to add a filter for the Update Source field from the Updates datasource. The filter condition you'd want to use would be DOES NOT EQUAL MicrosoftUpdate
You have the option of adding in other fields to the report (like Classification, Update Arrival Date, or Update Release Date for example).
Here is my report definition:
...and here is my result set:
That worked great. I was able to use PowerShell to grab the contents of the report and email it in HTML. Thanks!