Every Boarding pass has a code.. either a bar code/number, QR code, etc. No matter if you get your boarding pass from a last-night web check-in, from a counter with an AA staff, from a computer at the airport.. your BP will have a code. In that code you can have the Lounge Access permission (apart from having it printed in the BP itself). Passengers flying F go to the AC and the staff there scans the BP. They'll know if they can access or not. It's not that hard. And it doesn't involve a big IT investment either. Anyone with decent programming skills can do it. And I bet AA has a great IT department. Some people are dramatising it too much, you don't need to train everyone on AA or American Eagle if you add lounge access to F..

There won't be much difference in the amount of people in AC, in all honesty. How many of the people flying Domestically F, really pay for F ticket? From those people who do pay them, how many don't have Elite status in AA. Because if they do, they would enter the lounge anyways.