There's nothing in the policy (that I'm aware of) that says they HAVE to go by seniorty. I guess you could put in for a grievance. Worst they can do (and what they'll probably do) is say no.
I went through something similar at my barn on the city board. An opportunity to volunteer at another center popped up. A DDI managed to snag the assignment for one of his students, so I had a bunch of guys filev ethics complaints. Had them throw the word "union" in there. Made them go down the entire list asking if anybody wanted to go and nobody did, but it's the principle. Ops manager knows it was me and he's being petty by taking forever to approve my boot reimbursement. I'm going on week number 3 of waiting for it. I'm going road because I know the relationship with the ops manager is going to end up with me being terminated.