In the standard EBS functionality, there is no such option.
One method to accomplish that is to setup load balancing solution that directs all traffic to your apps server. Once the server is down, it can direct the users to a separate server that displays the message you want.
Regarding the maintenance mode, the web tier will display a message when a user accesses the login page.
ok I wondered about load balancing and guess I never tried logging in when the system was in maint mode.