They would have to be hitting a certain domain before you could change dns to 127.0.0.1 for the bots to attack themselves.
True you may be giving them what they want by your server being down but in some cases this can work when they see their bots dropping like flies. But its just a constant state of dns resolution and downtime if you had to keep going back and forth doing it so the site would basically be down for a while.
In some cases though Ive seen attacking botnets get the dns turned around on them to localhost and it stop the attack. Usually have to keep it that way for a day or so then try to go back to normal.
Ive only had to do it on managment clients with unprotected servers or clients who couldnt upgrade to the needed protection.
But if its not consuming your pipe, just overwelming you at the server level there is hope depending on how much time you wanna spend. Changing http servers, tcp tuning, and firewall scripts can help mitigate or completely block the attack.
You would also have to have fair amount of resources to do this. I guess it all boils down to is if this is a client or one of your own sites. Whether you think its worth the work or not.