AWS EC2 Autoscaling: Defining a master instance, which is never terminated

I am using EC2 with autoscaling and loadbalancing to host my webapp. To guarantee consistency between the EC2 instances, I only want to allow access to the administration interface from one instance, so all write operations are executed on this instance. The other instaces then periodically download copies of the changed files. So here's my question: Can I have a designated "Master" Instance, in my autoscaling group, which is slightly different (runs script for uploading files which were written to)? Of course this Instance should never be shut down, no matter what. All the other "Slave" Instances are indentical an can be created and terminated on demand. Is there some sort of configuration option for this or can I do this with a policy?