I just thought of one other possibility that would add a lot of gray to this.
If the loaders are employed by lets say John Smith, agent for Box Movers Moving Company and not by the actual moving company/motor carrier then a very strong argument could be made that the loaders are not employed by the motor carrier and therefore are not exempt from the wage and overtime laws. This would assume that John Smith is simply an independent agent and is not also a motor carrier or actually employed by the motor carrier his agency represents.
The same would apply to someone who had a business that provided loaders to the moving industry. Since that biz is not a motor carrier their employees would not be exempt from the wage laws.
When you get down to the bottom line there are only two questions that need to be answered to determine if the loaders are exempt.
Are they employees or contract labor?
If they are employees are they employees of a motor carrier?
If the loaders are employees of a motor carrier or contract labor they qualify as exempt. If not then they are covered under overtime laws.