The problem is a bit what everyone understands as s3. I associate this indeed also with an http endpoint on object storage. But the ceph plugin skips this http and talks directly to object store. I don't think you would like to operate on this http level. If I look at this page of ceph[1], it also looks like you do not want to get yourself involved in deduplication.
[1] https://docs.ceph.com/en/reef/dev/deduplication/
Moreover, following Filip remark about block deduplication, having any kind of deduplication that is not optimized for the email case (where attachments are always embed in slightly different documents) would make it ineffective.
Dovecot has this option to store attachments separately not? So I am not sure this is then still a problem.
Is it really worse bothering deploying a whole Ceph cluster for that ?
No you should not get ceph just for this. But ceph brings you nice redundancy, distributed storage. I am totally fan of it.