An FYI for anyone else searching - ESET ECA currently can't do this directly, but ESET UK support helpfully provided a sh script which can install the ESET remote agent silently, connecting endpoints directly to the correct ECA instance.
This worked for me - Meraki can deploy the script by wrapping it into a DMG package and deploying as a custom app.
ESET ECA can then install ESET sofware/licences etc, with everything being 100% remote.