No problem on Z fare
I normally book Z/D-fare tickets and show up at the airport when I´m ready but normally at least on time.
I never had any problems to get a seat when available. One time I actually downgraded to Y when slect was full, because I really wanted to go home.
This also works the other way around for later flights when I missed it. I have to say that I normally do not have any lugage to check.