We can add a custom property which says the server is under maintenance.
For example:
Go to your appserver console -> click on servers -> server types -> application servers -> server1 -> webcontainer settings ->webcontainer -> custom properties -> click on new and add custom property.
Give Name : com.ibm.ws.webcontainer.webgroupvhostnotfound
value : Server is under Maintenance, we regret any inconvenience caused
save this and restart server instance [server1]
That's it.. If your application is down, the error message, which you configured in custom property, will display to the user.