I think it would work very well if they had separate queues like this.
I keep admiring how well CX do it at HKG. 4 lines: IIRC First&Emerald, Business&Sapphire, Premium Economy&Marco Polo, Economy. Each line is policed with ID and boarding pass checked and stamped before boarding starts. DYKWIAs are consistently sent off to the correct line (and overly modest pax invited to join a "higher" line if applicable). Sure, CX have loads of space at their gates at HKG. Still, I think it's doable in Europe and on short haul too.