I would think its just a variation to the FMS conditions - I don't see it as odious - and my own view is that the Growlers will probably stay - there are some significant advantages in maintaining a short squadron within the Project/Plan Jericho constructs
Shornets would only be made available if they were surplus to reqs.
Again the nostradamic view I'm inclined to hold at this stage is that the Shornets will go and Growlers will stay on.
Once the Shornets are released, then its a NZG to State Dept issue. Assuming that this was an NZDF future force desire, then State could delegate how and what needed to be done to the Shornets in OZ prior to transfer to NZDF. There is no physical need for them to be sent to CONUS under those conditions
All of this is so far down the road that......... trailing sentence kicks in......
Taking my Nostradamus view I get the feeling the Shornets will stay on alongside the Growlers until something new comes along to replace them, that isn't the JSF...