As long as you like the black kilt AND recognize that a black kilt is not traditional highland dress, then wear it! For the future, keep your eye out for a more contemporary formal jacket to wear with the black kilt for black tie events. I do not know what men wear to the opera in Vienna, but if you wear the PC with the black kilt on the first night, then the PC with a tartan kilt to the opera, you will still have two distinctly different looks.

Wear what you want and enjoy yourself! I suspect that you will see a wide range of levels of dress at a UN event, except when it is white tie.