Here's a curious behaviour...
The client wants to redirect to another site immediately after login (don't ask why, please don't reply with "that's stupid", the answer would bore you). Under User Settings there's the option to set the landing page after login, so the solution is to make a page that's a URL link and set the landing page to that. Except, once you make the URL link page it doesn't appear in the list of "Redirect after Login" options.
Now the immediate, but inelegant, solution is to use a META redirect. Is there a good reason I can't choose anything but a vanilla DNN page as a target after login?