If they cannot handle the HN hug of death, I am not so sure if they can ward off a serious attack.
The idea - Realtime blacklisting via DNS - is not bad. But if the first impression I get is a page that loads very slowly, I am doubtful if they can implement it well.
Website is being worked, dns infrastructure is solid and working well. Sorry for brief response, a bit busy ;)
I hope you succeed. Filtering out bad actors via DNS is a good idea, you will have to be very careful about false positives, though. ;-)
I think a similar approach is already being used for mail servers to detect spam... but I am short on details, because the only mail server I have ever taken care of is the Exchange server at work, and Exchange is not all that proactive when it comes to spam.
The best anti-spam advice I could give WRT your Exchange box (I've managed those too) is to put another box in front of it to handle the spam filtering (Postfix + SpamAssassin + friends in my case, but you have many options), though IIRC even Exchange can directly use these blacklists nowadays.