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The way I can think of is using IAM credentials to allow modification of a Route53 record using the AWS CLI from the newly spun server. You will need to write a short script that you could push to the new instance either through EC2 MetaData or UserData fields.

If there is a demand I could try whipping up a CloudFormation template to do it.

Either create the ssh host key outside and set it inside the image or integrate the new host into your configuration management and report the sshd host key back and export it to the zone database. Of course your AMI could also just automatically populate the DNS entry via some shared secret or API ...

I don't know about EC2 but Linode provide you with a web based console you can use to set stuff up.