Few days back, I was tried to set up Maintenance page for one of SharePoint web application based on the below reference link which worked perfectly for me.
http://www.sharepointdiary.com/2012/07/maintenance-page-for-sharepoint-2010.html
Below are the steps I have followed
1. Create app_offline.htm file with the following content
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Site Under Maintenance</title>
</head>
<body>
<div align="center"><br />
<br />
<br />
<br />
<h1> The Site is under Maintenance </h1>
</body>
</html>
2. Place the app_offline.htm file in your specific web application IIS root folder where the web config file located.
3. Once you placed the app_offline.htm file in the directory, then the site will be under maintenance page without stopping your SharePoint web application.
