If I recall correctly, UA did this back in late 2020 at IAD for a IAD-ZRH flight.
No registration. Announcement: "Today we're going to board using biometrics...."
At the time I was surprised how this could possibly work (how did it recognize all passengers by their face?), and slightly concerned that everyone had to remove their mask.
Unlike OP of this thread, it worked fast back then, and the boarding line was moving at a decent pace, although slower than scanning boarding passes, IMO. I recall them having more than one biometric scanner running at the same time.