There is actually another way, more straightforward. As you can read in Laravel documentation, returning NULL from closure will make Laravel ignore particular request:
If the Closure passed to the down method returns NULL, maintenance mode will be ignored for that request.
So for routes beginning with admin, you can do something like this:
App::down(function()
{
// requests beginning with 'admin' will bypass 'down' mode
if(Request::is('admin*')) {
return null;
}
// all other routes: default 'down' response
return Response::view('maintenance', array(), 503);
});