As grrowler wrote, Qlink have been doing it for years.
Generally, what happens is at a predetermined time (ETD - x), the flight is called in the usual way at the gate, and the PAX are sent to the aircraft unless the Captain advises otherwise (problem with the A/C, etc).
When everything lines up, it works reasonably well, but it is very easy for the wheels to fall off if everybody isn't on the ball.