I had/have a similar seat assignment issue... booked a Y flexi SYD-AKL on codeshare.
DJ couldn't access seat maps/assign seats. NZ tried for me but said their system will not allow because its sold by DJ.
I quizzed DJ more today, as my flight is in 12 days... still no joy with DJ or NZ.
After initially blaming NZ, DJ said they do not assign seats on international short haul because they need to sight passports and therefore it was a "security issue".. when I explained that if that was the case it is a system limitation issue on their end to pre-enter passenger info etc. the agent didn't seem to want to listen. Also, said on the phone I am showing as "Gold" on the booking even though I’m Plat, so appears there is still misalignment with status recognition.
This is definitely a systems integration issue. As you all know shorthall INTL reservations is still Navitaire/New Skies system which requires lots of work to align with the GDS's in the likes of NZ. Also, Velocity alignment is still patchy and as others have mentioned is still very much in its infancy.
I think it’s important for DJ Agents to understand this and acknowledge it to their customers, rather than blame others or not understand how customers perceive this limitation...