There was an interesting system at Jakarta last week for a Malaysian Airlines flight. CGK has these mildly odd donut-shaped gate lounges, where entrance from the airport is from one side ("south") and entry to the plane is from the "north", but there's a part in the middle which is not accessible. So as soon as people came to the gate lounge and had their boarding cards processed, they were directed to either the left or right side of the lounge, based on their seat number (no, not just biz/eco, but actual seat number). Boarding was then done by first opening the left door for C/F and people in the back, then the right door for people sitting up front.

It seemed to work pretty well, but obviously you need a strangely shaped lounge for this, and I've never seen another airline attempt this, even at CGK.