Actually they do. I routinely do this when I am arriving from an international flight and have a long domestic connection. I show up at the lounge, ask them to put me on an earlier flight and if there's space they do it. In fact I did it on Monday, when I landed at 8pm and had a midnight flight to ESB. They put me on the 9pm flight instead. I don't know if it works with international though.

This is in fact better for the airline too, as they can sell the later flight to a paying customer whereas the seat you are taking in the sooner flight will most likely go out empty.