Caddy can already be used in a fleet configuration or behind load balancers if you share the $CADDYPATH folder with all instances in the cluster. Then it will coordinate the management of certificates and share them.
But now, Caddy supports any storage backend for this with cluster plugins. Here’s how to write one: https://github.com/mholt/caddy/wiki/Writing-a-Plugin:-Cluster-Support
These can now be added to the Caddy website and made available for download as part of custom Caddy builds.
Let me know if you make one! We’ll also link to it from the CertMagic README.