I'd have to read your supplement, but I think you could file and win under the preferred jobs language. Check with your BA to be sure.
I'm a steward and I helped an employee who was an unloader, that was being moved to the prelaod, file a greivance and win due to the fact management was not moving other quailified lower seniorty employees first, where available.
Management tried to stonewall our request the same as you, you will probably have to file and see it through as management will most likely not cooperate.
Bottom line is this(unless your supplement is different). As long as there are lower seniority employees who are in your work area and are qualified and available to do the job they are sending you to do, you should prevail. The key part is qualified. If you are the only employee in your work group that is qualified to load then you may be out of luck. BUT, don't let mgt tell you you are the only one qualified if they regularly send others up front to load. If those employees can load an area by themselves, they are qualified. If you have little to no experience loading, then anyone in your work area is just as qualified as you and mgt should be moving employees based on seniority.