Maybe I'm missing something here, but why would you not be able to use MC on a townie if they aren't on-screen or in front of you? You can get to the filtered sim list on the lot where they actually are (where the prom is taking place) or by way of City Hall or an in-game computer. They won't show up that way if they are off-screen hibernating in their own houses or as homeless sims do, but then presumably they wouldn't be at the prom having their picture taken if that were the case either.