Local supplements cover these sort of jobs.
In my local FT unassigned cover drivers (meaning they never bid a route but are guaranteed 8) reserve the same right under the contract that bid drivers do. 9.5 opt in, 10 hour OT grievances, and 8 hour requests apply the same way they do drivers with bid routes. They are on the same center seniority list as drivers with bid routes because they are within the same classification. Start times are in seniority order regardless whether the driver has a bid route or not.
Unassigned cover drivers pick open routes or split cars by seniority.
Needs of the business can be used a maximum amount of 2 times a month by the company in the center (not always enforced, depends on the relationship between the the employees and management).
Also, national language allows you to use the local supplement (or national) if it is more beneficial for the member.