Might be better to go in the reverse direction - get them to store their DNS records under revision control and then when changes are committed/pushed they are deployed.

I wrote something like that for Amazon's rout53, https://dns-api.com/ but it wouldn't be hard to wire up a git hook with APIs from DNSMadeSimple, Dyne, or similar I'm sure.

(Of course if you don't host your clients DNS then it might be a fight to get them to change. But if you do it might be worth a think..)

I thought about that, but some of our clients use 3rd party contractors / internal IT to manage their DNS as well so getting those entities to use it would be hard. Eventually that would lead to mismatched records.

The current way, I can actually monitor DNS changes to clients without them having to change anything they are doing.

I have thought about a syncing type of feature where you'd be able to schedule a one-time or recurring sync for a particular zone or group of zones.

I guess it's good you've got a solution that works then !

If you had the option the simplest way to poll for changes would be to do a hourly/daily/weekly zone-transfer. But many DNS-hosts disable those for security reasons.

Yeah thats the trouble. The big providers disable zone transfers so you have to hope for an API. A lot of providers don't make one available. Besides the ones I have up there, there are only 2-3 more that provide a workable API which I'm planning on integrating soon (Dyn & Azure).