Similarly, I created a new coreos droplet just to play around, neglected to add the cloud_config yaml configuration. I couldn't find a setting to add it to the droplet afterwards.
With cloud-config, you're basically expected to be using ephemeral (or nearly-so) instances, particularly within the context of an autoscaling group or equivalent. The lifecycle is supposed to go "[scale up], provision, configure, start services; [crash or scale down], terminate, repeat." CoreOS adds to this soft reboots for upgrades, but definitely still assumes cattle, not snowflake, instances.