Assuming OP does gain SQ*G before travelling, they should still be entitled to gain lounge access simply by showing their KrisFlyer Gold card. Or would they be turned away because the SQ*G number is missing from the booking.....?
It would be unfair IMHO if they were denied access in this circumstance (holding a NZ award ticket in Y and being *G with a partner carrier). This situation seems a perfect example of why retaining use of the FQTS field would be useful.
OP. Once attaining SQ*G, you could call Air NZ and provide your KF details, and ask for a manual allocation of a 'frequent flyer' seat(s).