I'm doing some preliminary work on one. But realistically, it's a lofty goal and it'll be hard to get going. My priorities are different and so I'm taking some different design paths than datomic as well (ie, no datalog).
I have done some searching and have not turned up anything. The functionality that datomic enables is really intriguing, but I have trouble bringing myself around to using a non-open source bit of tooling.