I'm not a lawyer (yet), but I think you can't sign away your legal rights endowed by the state. It's like me signing away my right to free speech...couldn't do that even if I signed in blood.
Probably a collective bargaining agreement exists, but I don't think this was the case.
CA law is as follows:
"10. Q. Can an employee waive his or her right to overtime compensation?
A. No, California law requires that an employee be paid all overtime compensation notwithstanding any agreement to work for a lesser wage. Consequently, such an agreement or "waiver" will not prevent an employee from recovering the difference between the wages paid the employee and the overtime compensation he or she is entitled to receive. Labor Code Section 1194"
http://www.dir.ca.gov/dlse/FAQ_Overtime.htm