It might be more useful to return the IP address of whoever made the DNS query.
This could trick the computers that make up the botnet to either attack themselves on the public interface (more resource-intensive than trying to DDoS your own loopback), or even better, their ISP's resolvers (it would force the ISP to do something about it).
With the recursive nature of DNS, I imagine that could get a little hairy as the DDoS'ers would then be targeting whichever DNS servers they were using.
This could trick the computers that make up the botnet to either attack themselves on the public interface (more resource-intensive than trying to DDoS your own loopback), or even better, their ISP's resolvers (it would force the ISP to do something about it).