Back to the OP, in our center when the company knows they are going to have extra drivers, and especially when they are trying to train new hires, they will put out a sign up sheet for anyone that would like to have that particular day off. You must come in or call in the next morning and they start at the top of the list by seniority and allow off until they get the drivers on the street that they need to train. Have not had a problem with that yet.
I will say that seniority does not mean you can force someone to work and you get to go home! If the company puts you on a route as a cover driver because you know the route and lower seniority cover drivers are laid off for the day, your gonna have to work!! No matter how much you bitch about seniority!!