Simple and to the point. Truth be told I've always wanted something like this for my own domains, so that I can point to a home machine, but was too lazy to set it up. Thanks for throwing this out here!
Initially it was put together as a learning experience, but I can see that it is useful enough to remain live indefinitely.
I can imagine several way of monetizing it, such as adding the MX/SRV/TXT/CNAME records, etc. But the one that seems most useful would be allowing custom domain - and sadly I can't see how to do that easily.
(I can't imagine somebody would trust me with their root-domain, basically. And having *.foo.example.com seems like it would be long enough to spoil the utility.)
The “root-domain” is the domain name with a single zero-length label at the top of the hierarchy. What you're referring to is a zone.