Pay $9 for a subscription at http://www.expertflyer.com. Search for your flight and see the Z availability. Set up an alert to be notified when Z opens up. When you get the email, call SMS and ask them to if they can process your upgrade as you noticed Z is available.

You may want to manually check expert flyer as it only checks for availability periodically. For example, I was trying to get upgraded on ZRH-ATL later this week. I saw that Z was full and setup an alert. A few hours later, I checked and Z had 2 seats. Called SMS and got the upgrade processed. About an hour later, expert flyer sent me an email that there was availability in Z.

Good Luck.