I'd suggest extending the idea to non-responsive sites as well. Instead of submitting a link to a company's homepage when they're being DDOSed or are otherwise unavailable, submit a link to their status page if they have one or, failing that, use a third-party indicator. Off the top of my head I can't think of a third-party indicator that would capture point-in-time availability, but a manually crafted URL like http://isup.me/example.com?1970-01-01T00:00:00Z would get the point across just fine.