Ah this is so close. If it could have a s3 backend for storage of the email blobs (should be trivial since the email files are just key values from email id to email bytes) one could use it as a backup tool, too.

Author, is anything like that possible/planned? (the index itself can stay filesystem only)

I am the author of Bichon. Long-term backup and permanent archival features are not currently on Bichon’s roadmap, but you’re very welcome to open a GitHub issue to share your idea. Bichon is entirely driven by user needs, and any good suggestions or plans can be adopted and implemented.