CityRail in Sydney tried that on in 2004 and again a few years ago during EBA negotiations. They'd let driver numbers decline to the point the entire network was dependant on Driver's and Guards doing OT just to man a normal timetable, yet alone special event services. Come EBA time and CityRail trying to shaft their crew, they simply said "We've had enough OT for now, thanks" and the network ground to a halt.
There is no legal definition of "reasonable OT", and the courts have ruled your RDO's are your own, you cannot be forced to work an OT shift on an RDO. The ATCO's should simply spend their days off with their family and let us wear the delays. ASA will cave, and cave quickly!