Are you moving to Constellix on purpose or are they migrating you? If they are migrating you I’d consider a move to a larger supported provider instead (Cloudflare, Route53, Google Cloud etc). We don’t have any plans to implement a Constellix provider simply because there are thousands of possible DNS providers all with their own API. We would potentially implement it if it was added to the Posh-ACME project (where we get some of our DNS providers).
You can script DNS add/delete using your own scripting (in any language): https://docs.certifytheweb.com/docs/dns/providers/scripting
I’d personally recommend looking at running an acme-dns service as this lets you have a least-privilege DNS update mechanism. Although it’s a little complicated to setup at first, it’s a good system. I have considered whether we could provide a managed acme-dns service for customers but we haven’t had any requests for that yet.