If I'm incorrect and there are merits to it, please enlighten me, but everything I've read about OpenVR (and seen in their git repo) seems to only be an accurate name for where the meaning "Open" == null.
They should have called it ValveX. It's the best bad option among worse options (and that's probably why Valve won't put more effort into opening it up until the market matures), but it's not an open standard.
Most implementations are closed source binary blobs. Of course there is Mesa, but that is is also a 3rd party implementation. No one would prevent you from building an open source OpenVR implementation.
Anyone can implement a driver, they have example driver code available. The documentation is here: https://github.com/ValveSoftware/openvr/wiki
The purpose of OpenVR is to create a common API for HMD drivers, so that other platforms can implement their own proprietary equivalents of SteamVR or what have you, and everyone's HMD drivers will be interoperable.