Something like Rclone and a cron job, or else s3 mounted via FUSE, could possibly bridge that. Of course then you have to worry about reliability of the bridge...

There’s support via Barman cloud - we use it for azure at work but s3 and others are supported iirc

Mounting S3 with Fuse is not stable or performant enough at scale for backup storage

I can understand why it'd be preferable to avoid such a bridge layer, and indeed I too would rather just have a transparent view of what's going on at the protocol level.

Stability and performance at scale sound like implementation specific properties though. If you've tried this, I'd be curious to known about the specific issues you encountered.

https://github.com/Barre/ZeroFS Should do a great job at this.