Er, can't you just edit the app_offline file to suit? As long as the fie size is at least 512kb you can do what you want (below that size it may not display).
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xml:lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>[TITLE]</title>
<style type="text/css">
* { color: white; font-family: tahoma; margin: 5px; }
</style>
</head>
<body bgcolor="323232">
<h2>Closed for essential maintenance.</h2>
<p>We'll be back up and running as soon as possible: sorry for any inconvenience.</p>
</body>
</html>