Be aware that Z availability is dependent on your routing, whereas ExpertFlyer only lets you set alerts for individual flights, so unfortunately EF alerts are not useful for connecting itineraries.

And I would agree that Z availability does not tend to open up close to departure, at least not the way that Delta upgrade availability sometimes does. The Z fare class is used by KLM for deeply discounted cash business class tickets, so unless something goes very wrong in their predictions, they're typically not going to want to be offering to sell deeply discounted cash business class tickets close to departure. Whereas some airlines, including Delta, don't mind opening up seats specifically for upgrades if a flight is lightly booked in business.

I haven't monitored trends closely but I wouldn't be surprised if there are some instances of additional space opening up as you go from 11 months out to 6 months out or so (similar to how you can sometimes book tickets 6 months out for cheaper than you could for 11 months out, because the airlines don't start carefully monitoring flights and offering discounts 11 months out). But within 6 months I wouldn't hope for too much new Z space opening up.