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Not sure .. but I don't think you can display content if the site is turned off.
An alternative would be to make a file called index.html with html content stating that the sites down for maintainance. You could put a few highlights or whatever of the site in it.
This page will load instead of index.php. If users have other pages bookmarked and go to them .. they will still get the standard "Site Closed" page though.
When you're ready to go live again .. change the name of the index.html to index2.html or whatever (or just delete it if you don't want to use it again) and then index.php will take over again.
Heres a very simple one I made for my site
http://thaiexpat.org/index2.html