Any way to auto-failover internal web server to alt.?

[Guest]

[sagor]

Before I spend a lot of time trying to figure this out on my own, I thought I'd ask.
What I would like to do, is have EFW as my basic firewall/router. On my LAN, I already have a web server with a few sites on it. Now, if I take the web server down for maintenance, I'd like EFW to auto detect that the IP is "dead", and change a rule (NAT or proxy or otherwise), to re-direct incoming WAN HTTP requests to a backup web server (different LAN IP), where I can post simple messages that the main server is down, and why, etc. 
I've seen items (including feature list) refering to a backup WAN failover rule, but nothing about a backup failover LAN rule.
So, is this do-able? Or am I looking at something too complex?.
Thanks