This is exactly what I had dreamed of except without being aware of the session extension... I put it off because I thought I would have to create a layer opening sequential partial sqlite dbs and somehow filtering to the most recent version of each record in-between compactions.

It sounds like sqlite sessions handles the hardest part and you've sanded down some of the rough edges while implementing the glue bits for s3 (generously licensed MIT); thanks for the heads-up!