OP,
I had a similar itinerary to MEL with the second leg from NAN to MEL in economy. I kept monitoring the economy flight and when I saw saver business class open up I phoned Alaska.
The agent said the same thing, she couldn't see the space. I told her if you try to make a separate booking for just the NAN to MEL the J space is indeed there.
She did see that and said the best she could do was put in a request for the economy leg to be upgraded to business. She put me on hold and about 10 minutes later returned to the line and said to her surprise the request was approved. In fact, since exchange rates worked in the USD favor I was given a small refund as well of the taxes!
Idk if it is a HUCA situation until you get an agent who can do it, but it's definitely doable IMO.