Joeydknecht,
Since you are talking about a U.N. event, you more than likely will see people there in various ethnic attire. The invites for UN Association Balls I've attended in the past said 'Black-tie or national costume' if I recall.
I would wear your tartan though, the all black look is either too Hollywood, or too conformist not sure which! I also see you mentioned a belt buckle along with the PC and waistcoat? As is often discussed here, one generally doesn't wear them both. Have fun!