Getting seats through Mexicana is VERY difficult since they can only book 10 months ahead.
Try Aeroplan, they can book Air new zealand's inventory 350 days ahead. More chances to be the first ones to grab those high demanded seats.
I just grabed a couple of seats for Oct 28 07, just need to decide if i want to go with the 777 or 747 business class. I have until tomorrow to decide before the res agent put my Aeroplan miles through.