Huh. To me, "go out and get the latest release automatically" is terrifying; you should be testing new releases before you deploy them.
It would be simple enough to extract the AMI numbers from FreeBSD release announcements though; the main reason I encourage people to use the Marketplace AMIs is that the AWS Marketplace provides useful statistics about how many instances are running.
Maybe "release" isn't the correct term. Ubuntu publishes new AMIs with the latest security patches quite regularly. When I spin up a new instance on say 14.04 I want it to boot with a fully patched kernel.
IIRC The EC2 FreeBSD applies all new security patches and reboots (if there were any) on first start.
The nicest experience I've had is with the Ubuntu community AMIs. Ubuntu keeps a listing of their official AMIs[1] and there is a JSON api that they provide so you can fetch that information programtically.
[1]: https://cloud-images.ubuntu.com/locator/ec2/