Actually, I realized I can get by with a redirect after login for now (just a simple modification to include/checklogin.php), so I'm happy to hand this task over to tzvook
I downloaded both modules from the repository here.
The Startup module seemed to do exactly what I was after. Maybe it's fairly easy to update to 2.3.x compatibility. Which do you think is best suited for upgrading, Startup for 2.0.x or Startup for 2.2.x?
And I assume there's really no way to do this through a manual process either, i.e. register the user centrally and add the notification to the user, without actually logging in as the user in question?