I'm not an Air New Zealand pilot, but will comment anyway
Unfortunately, your only chance of a look in the cockpit is when the aircraft is stationary on the ground, i.e. before or after the flight. The crew are generally too busy before, which just leaves after! You could ask cabin crew to request a visit after landing, or just ask as you leave the plane.
I foresee two extra problems for you:
1) The Captain and F/O will be tired after a long haul flight and will be keen to get home or to a hotel.
2) If the aircraft is a 747, you will have to blag your way upstairs through Business class (unless you are there already?).
Despitethe negativity of this post, I still recommend that you give it a go. I spent a very pleasant half hour in the right hand seat of a BA RJ100 at Monpelier after a diversion from Nice due to thunderstorms. The Captain showed me the systems, and how they compared to a C-152 (not very different really). Meanwhile the First Officer, who it turned out went to the same flying school as me, was supervising the refuelling, most of which went over his shoes.
Good luck,
TG.