With UA's direct routing restrictions and NZ's withdrawal from most (all?) Sth Pac Islands - NthAm routes, I don't think there would be much leeway for any stopover other than AKL.

Try checking the various carrier's websites out for cheapie returns from AKL to somewhere SouthPac return as a seperate side trip.
PacificBlue, PolynesianBlue, Air Pacific, Air NZ, Air Calin, Air Tahiti Nui would be the logical places to start.