While I see that it may be useful to have zone data in an sql backend, I don't like the idea of plugging a mission-critical service such as a dns server directly to an sql database. A dns server has to be as simple as possible, with as few dependencies as possible. Serving zone data directly from an sql database increases the complexity of your system and adds new points of failure, what is especially undesirable in the case of a dns server.
Well you can always write a small program to read out the data from a sql server and create zone data in the format required by your dns server :-)