At a job in Minnesota, we were paid a "production bonus" for working over 40 hours / week. Bosses were extremely clear about never calling it overtime, because they legally couldn't.
The reason? We salaried, not hourly employees, and overtime is a legally regulated thing for hourly employees. Of course, we were paid the equivalent of our hourly rate, not hourly + half, which might have had something to do with it, but working over 40 was always optional, so noone complained.
I'm not a lawyer, but that's what we were told at least.
In California you can't voluntarily waive overtime if you're entitled to it. And whether you're entitled to it is a question of fact that can't be changed by calling it something different. California law is very clear that you can't contract your way around public policy.
The reason? We salaried, not hourly employees, and overtime is a legally regulated thing for hourly employees. Of course, we were paid the equivalent of our hourly rate, not hourly + half, which might have had something to do with it, but working over 40 was always optional, so noone complained.
I'm not a lawyer, but that's what we were told at least.