This is (almost) exactly what Goatfish[1] does, except it uses SQLite, and can create SQL indexes on any arbitrary field. I needed a schemaless, embeddable store, so I decided to go with that, since Postgres already has the hstore. It's still very simple and preliminary, but it works, and it's very useful for prototyping.

I'd like to develop it some more, if people found it useful and started using it.

[1] https://github.com/stochastic-technologies/goatfish