ZoneEdit works through their own interface; you can set up ns3 and ns4 to point to ZoneEdit servers but you'll have to make additions at the ZoneEdit site. I don't know how complex it would be to write a script to manage ZoneEdit using curl, but it may be possible.
One way to manage redundant slave DNS is to have another DA server in a different location, and to use the DA Multi Server Setup option. Another way is to have another server in a different server (not running DA) and use Master2Slave, found elsewhere on these forums.
Contact me by email if you'd like information on a commercial slave DNS solution that will automatically slave DA servers.
Jeff