Per discussion:

* I had a few concerns about doing that - largely with respect to special cases and unforeseen non-standard/exotic configurations. Using a configuration provides some flexibility to accommodate. Per discussion, we will not worry about this.

*  I tried using Ruby's IPAddr #reverse method. PDNS did not like what ruby was spitting out of reverse(). For instance, given 192.168.100.0/24, #reverse returns 0.100.168.192.in-addr.arpa. Dropping this into the domains table as a zone did not work. It needed to be 100.168.192.in-addr.arpa. Since ruby does not shorten for larger classes, I had to get into parsing the output of reverse, which introduced assumptions and the possibility for convoluted logic later on to deal with unexpected configurations.  Basically it felt like the wrong path to be going down, which could be obviated completely by just using a configuation. 

I suggest a compromise:  Check the environment to see if reverse_dns_zone is present. If it is, use it. If it isn't, calculate it. That allows odd cases to be easily accounted for, and easy cases to work by default without pestering the user for another config item. Does this work for you?