We're working on an implementation for SirixDB with currently a slightly different syntax (for instance "=>" to dereference object fields). We're also supporting all kinds of temporal query enhancements (point in time queries, time travel queries...):
https://github.com/sirixdb/sirix
The query engine used is developed here (core implemented by Sebastian Bächle and his tudents). Ideally the backend can be any other data store as well.