In BOS, my experience is that it is nearly completely random as to who gets the upgrade once you're under airport control. Many of the agents can't be bothered figuring out the rules, so they upgrade whomever the person is that happens to be standing in front of them, to minimize the work involved on their part. This is, of course, not always the case, but I have seen it happen many times.

They will also tell you that the flight has "checked in full", even when some people may not board because they checked in online and decided not to travel. In this case, again, the upgrade is often given out randomly, and once you board you have no chance of getting it (there would be work involved in going on the plane to find you).

In other AA cities, I have seen much more predictable behavior that attempts to conform to the rules, but I wouldn't get my hopes up in BOS. I think your best shot will be getting on the good side of the gate agent.