It sounds to me like a PHP, Perl, Python, ... script could be used here.
After thinking about it a bit I'd probably suggest a monitoring service be setup to change the internal DNS entry for that external website when it goes down.
The new DNS entry would point to another site.
I've done this manually in the past when a hobby server had to be shutdown for repairs or whatnot.
If I come across anything more useful I'll post back.
Hopefully someone with more knowledge will post soon.