You should have no problem booking on nwa.com and using miles to upgrade as long as there is Z class available. I booked DTW-LAX-SYD in October in M class on nwa.com in order to get the cheaper fare and was able to upgrade without any issue, it just took the agent a while to process it.

That said, the next 30 days do not look good for Z class availability at this time. Once you get into August there are a few dates available, but it's very hit or miss.