If it is possible to allow Rage4 to provide secondary DNS it could potentially allow customers to use multiple providers from a shadow primary. This would offer greater flexibility though its likely it would be at the cost of geolocation. Caching and serving records would allow virtually any record to be served. It would also allow the DNS records to be maintained in house.

This type of configuration is supported by at least two major providers with anycast services, though it is not a very common feature.

As you consider this I'll add a couple more items:

1. Add a migrate button so customers can import an existing zone and start using Rage4 as primary. This is an excellent migration strategy.

2. If you are concerned with the security of zone transfers you could implement TSIG as part of the DNSSEC rollout.