Would it make sense to build an ASG for which the EC2 instances register to a network load balancer and have the new instances join the cluster by feeding it the NLB address during the CRDB startup on the newly created nodes?
In the ASG all EC2 instances get their IP addresses dynamically so the only thing I know that is always is pointing to the running instance[s] is the load balancer. If this is possible, the next challenge is to find out in which zone and region the newly created EC2 instance is started to put that into the locality flags.
If this approach is valid for scale-out, next thing is to find something to handle the scale-in. I think this should be done by one of the remaining nodes by monitoring the nodes that are registered in the NLB and compare that to the nodes that the database is aware of…