I was in similar situation and end up using apache in front of tomcat.
Ie when tomcat is down from some reason, Apache intercept the 503, and use httpd config
Error Document 503
to redirect to your own custom page. Some details can be found here apache tomcat 503 custom error page.
503 is quite serious compared to 404 etc, most of time there are some un-recoverable error which crash the service/app, in our case heap-memory issues.
I would also be quite interested in anyone else comes up with how to redirect 503 from Tomcat alone.