Simple solution is quite easily implemented. The server knows how many users are logged in at any given moment, and propably some value can be set, after which (auto)login for non-members redirects the user to the page I mentioned before...

The "correct" way to do this would of course be to monitor the actual strain on the servers - much more difficult, although possible.

Edit: Fencer of course knows best. I have no insights to the software. :)

Edit2: 10-30% was a rough estimate, or as some people like to call it, a guess. :)