I split with OW all the time, earning on QF, using Emerald status on BA to get better lounge access than on QFF. AFAIK it works because QF bills BA for the lounge access as Emerald status, even though it credits to my QF account with FFPs (this was explained to me some time ago).
Is *A different? On this basis, NZ would bill the other *A carrier for lounge access using its card, but credit airbucks/status credits to Airpoints.
In any case, even if NZ didn't allow it, it wouldn't make a material difference to NZ lounge occupancy. The end of the VA alliance might make a small difference at the international lounges, if VA didn't pay NZ for its status pax to get access to its lounges, although it may yet still do so for VA flights at least in some locations.