To back up Juggernauts obvious knowledge on the topic, I just had another look through the IRB website and found the eligibility section.
The "One Union" rule, as it is known, came into effect on Jan 1st 2000.
As soon as you turn 18 you can only represent one Nations TopXV, 2ndXV or 7's team if you are present at their game.
That means that even if Nacewa hadn't got onto the field for Fiji he still wouldn't be allowed to play for NZ. It doesn't make a difference if you're a starting playing OR a reserve.
The MINIMUM Fines for Eligibity Breaches are as follows:
100,000 pounds sterling - For a Union that is part of the IRB council
25,000 pounds sterling - For all other IRB member Unions
Now thats some serious pennies per offence!
Thats probably why, even though Nacewa is the man, the NZRU hasn't risked putting him in for the AB's.
Players can be eligible to play for a nation if:
A. They were born there, or
B. They have a parent or grandparent from that country, or
C. They have lived in that country for 36months
The only way a player could now play for 2 countries is if they had lived for 36 months in the 2nd country they wished to play for prior to the 1st of Jan, 2000.
So thats rather unlikely nowdays.
All this info, in a longer form, is here:
http://www.irb.com/mm/document/lawsregs/0/...tion8%5f883.pdf