Can you change the polarity on a mac charger?

Hi,
At the end of the month I am flying with Eva air, and they have power outlets in the seat, but am under the impression that the plug is only 2 pin USA style with minimal space, (no room for adopter).
Does anyone know if i was to us a standard C7 Cable would it matter if i were to can you change the polarity, would to damage the charger?

If they're providing mains voltage, I'd see no reason why the long cable and an adapter wouldn't work.