Have an almost-static replacement index ready with just enough stuff for a php "give us your email address and we'll send you invites [and a discount or some other bennie]". No CSS or JS on that alternate universe page. If things get nasty you'll want this to be as few keystrokes as possible, because it's possible to trash interactive response in the shell.

Maybe make a pass through your normal site with YSlow if you haven't already, but your site is super fast right now (2009-01-20 17:20pm PST), which is good news.

Also, if you're concerned about it still and you have another hosting option available, change your index.php to <? header("Location: http://www.temphosting.example.com"); die(); ?>, which is probably the smallest amount of cpu you can involve without getting dns moved.

Good luck.