Qantas do do "Flow Forwarding" during peak times like Friday Evenings on main routes such as MEL-SYD when their aircraft are heavily booked.

They see all the seats as equal and will pro-actively offer earlier flights, even using the quick check machines.

Once cannot count on this and you can be sure that if sales are slow, then those on inflexible tickets will almost certainly travel as booked. (If this were not the case, one could book the cheapest flight at say, 8pm, front up at 5 and take off.)

FWIW, DJ actually provide this as a listed benefit for their Elites.