Update
My initial answer still applies concerning the question as such (see below), however, given you actually need to do this to be able to then move that AMI to [...] a different region, you will be pleased that AWS has just released Cross Region EC2 AMI Copy to address this long standing feature request:
AMI Copy enables you to easily copy your Amazon Machine Images between
AWS Regions. AMI Copy helps enable several key scenarios including:
- Simple and Consistent Multi-Region Deployment - You can copy an AMI
from one region to another, enabling you to easily launch consistent
instances based on the same AMI into different regions.
- Scalability - You can more easily design and build world-scale applications that
meet the needs of your users, regardless of their location.
- Performance - You can increase performance by distributing your
application and locating critical components of your application in
closer proximity to your users. You can also take advantage of
region-specific features such as instance types or other AWS services.
- Even Higher Availability - You can design and deploy applications
across AWS regions, to increase availability.
Initial Answer
Now I want to backup that AMI to a S3 bucket. Is this possible?
While Amazon EBS indeed provides the ability to create point-in-time snapshots of volumes, which are persisted to Amazon S3, this operation is outside of your control and entirely handled by EC2, see the respective FAQ Will I be able to access my snapshots using the regular Amazon S3 APIs?:
No, snapshots are only available through the Amazon EC2 APIs.
You can achieve your goal by following Eric Hammond's elaborate article Copying EBS Boot AMIs Between EC2 Regions, which guides you through all required steps (quite some though).