[eluser]TheFuzzy0ne[/eluser]
I can think of perhaps 10 different ways off the top of my head. The simplest being to just set up a route at the top of the list:
Code:
$route['.*'] = 'maintenance_controller';
Obviously you just need a maintenance controller, and you just comment out the route to disable it.
Note: I'm assuming that you don't need admin access to the site during maintenance mode, as that route will just shut everyone out. If you do require admin access, you could add a bit more logic to make up for that.