You could wire up two DPDT switches (or one 4PDT) to swap the A & B buttons round for each player, a'la a yoke flip switch.
It's not software based but still a lot easier than swapping terminals over, especially if it's only for one round.
Edit: In case it helps, you would wire the buttons up similar to this DC motor diagram:
You could wire this in with some male & female QDs. It doesn't really matter which way round you do this but for example, connect the input wires that usually go to your buttons to the 'M' side of the circuit. Then connect wires from the center pins back to your buttons. Your GND wires stay as they are. Flipping the switch then reverses the inputs of your buttons.
The above is a DPDT switch. A 4PDT switch would be the same but just with two circuits in parallel. Benefit of the 4PDT would mean it only one button to press/switch instead of two.