Possible to rsync to Amazon Glacier?

Not possible with rsync. Glacier uses its own protocol for uploads, and requires that catalogs/indexes of the backups be retained by the client software that uploads the files. Once you store an "archive" in a Glacier "vault," the archive is as opaque to you as if you were examining a physical backup tape with your eyeballs to see what content it contained.

Glacier is an excellent service, but not a match for this application by itself -- it requires client software that plays an active role in backup management, since it isn't possible to directly "compare" local files with what was stored in Glacier.

The Glacier interface from S3 comes closer to to this, since when you get to Glacier via S3 you can still see object metadata, but again, purpose-specific software would be needed for such backups.