My previous announcement that there will be a Berkeley DB backend was too hasty. I ran into some (for now) unsolvable problems. I posted them into the official Oracle forums but seem to get no reply there:
https://forums.oracle.com/forums/thread.jspa?threadID=2307258&tstart=15

The final nail in the coffin was, that the advertised increased concurrency was not present. In my tests it performed even worse then SQLite in WAL journal mode. That and the perceived instability (I had a database corruption once) shine a pretty bad light on Berkeley DB. Maybe the SQL layer for Berkeley DB which I used is not stable yet?

On the plus side the tables I had to denormalize cause a significant speed increase for SQLite as well, so all this work was not for nothing.
Maybe I will revisit Berkeley DB in a few months/years.