phranque - 12:40 am on Nov 2, 2012 (gmt 0)

I'd get a custom 503 unavailable page up there asap

absolutely!
my bad for not thinking of that when i replied previously.
make sure it actually returns a 503 status code.
same as a said above except you will be monitoring 503s instead of 404s.

503 Service Unavailable:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.5.4