We use weighted A records for this purpose. We basically have two weighted A records, one pointing to the load balancer and the other one to the static maintena

We use weighted A records for this purpose. We basically have two weighted A records, one pointing to the load balancer and the other one to the static maintenance site on S3. When we want to switch sites, we just flip their weights (they are always 1 and 0), hence enforcing 100% redirection. It's not a perfect system but it works and DNS propagation is usually done within 15 minutes (haven't tried modifying the TTLs). Haven't been able to come up with anything simpler than this.