Hey Erik -
As you've discovered, tabcmd doesn't support this directly. However, the information you need can be found in our system tables and you can create a custom admin view to pull it out.
Read through this article and then connect to "our database", then fire the following CustomSQL - it should get you what you want:
SELECT u.Name, Count(w.*) As "Number of things owned" FROM _users u INNER JOIN _workbooks w ON u.id = w.owner_id Group By u.Name