Waitlisting is possible...
After reading this, and having an upcoming flight from GRU -> ATL booked in Y, and also having been told by NWA elite line that there were no Z class seats available, I called the Delta Skymiles line. I explained to the person there that I still had a few of the systemwide upgrade certs available and she basically took one of the cert numbers and added me to a waitlist so that if and when a Z class seat comes available, I should get upgraded. We'll see what happens...