I suppose that you could use java and cookies to ask the visitor if they'd like to save the IP to their favorites and once they make a selection use a cookie so they're not asked again or for so many days or visits. Other than that if the domain can't be resolved due to a DNS outage, there's no way to redirect them to the IP. It's very rare for DNS to malfunction on a server and usually if it does there is another cause that will take down other services as well so httpd will follow soon.
The best solution is adding more nameservers for the domain by using third party nameservers.
