I missed this in the deluge of news coming out of Unreal Fest last week. Marcus Wassmer during the State of Unreal keynote:

MetaHuman has been adopted by some of the most successful games in the world, and is rapidly becoming a standard for digital characters. That's why, today, we're announcing the MetaHuman DevKit, opening up our rig logic and MetaHuman DNA toolsets under the MIT license so that you can deliver MetaHumans in any game, any engine, any platform.

For now, MetaHuman DevKit starts with the OpenRigLogic repository, which contains RigLogic and DNA as native C++ libraries with Python bindings. The website seems to indicate that more, if not all, of the technology powering MetaHumans will eventually be open source.