No, I have not heard of an A or B list, but there is some leeway, and it would make sense to have two lists if employees or others have a different order.

But I did have an experience once in DTW where the agent "re-arranged" the list. It was a while back, so things may have changed some. But I was connecting in DTW and was U/G on the first leg. Got to DTW and asked at the Gate if my U/G had cleared or where I was on the list or something of that nature. The agent got one of those funny looks on her face. And said, "You are second, but the first position is a silver, so we are going to move them back to where they belong and put you first" She also happened to have one seat open up and printed me the boarding pass.

The agent said a possible scenario was that the silver had misconnected on an earlier flight and the previous agent was trying to be nice and moved him up. However, she said that was not allowed and they had to go back in the que where they were supposed to be.

So, apparently there is a way to move people around on the list, but ultimately the gate agent has the final say.