Hello,
I agree, OrientDB looks very good and I'd love to have it supported in Hibernate OGM as well.

But the core team is small, and the initial priority was to have on NoSQL database for each main "family" to verify flexibility of internals; currently our focus needs to be to do some refactoring to adjust to all the lessons learned, and we're quite busy with that. Would you like to help us integrating OrientDB? Adding a new database - especially when we already have support for another graph database - should be relatively easy, and I'm happy to give you all needed pointers and help as I can.