Besides the Megagames of the previous post, and the National Security Decision Making Game (a U.S. group that runs large-scale international simulations at gaming conventions) I'm not aware of anyone else doing this sort of thing for recreation. That's excluding, of course, model U.N. groups in high school and college, some of which (such as the University of Chicago's MUNUC and ChoMUN) make a point of producing high-quality, innovative simulations. But why so little interest in this sort of thing after college? Here are a few thoughts:
Preparation time: Putting together something like a Megagame or a scenario for a NSDMG takes a long time. There are no published supplements, and everything is pretty much done from scratch. College students might be able to spend this kind of time, but it takes a devoted hobbyist to do so out in the "real world." There's also the question of preparation for the individual participants; there are frequently background papers or rules to read beforehand (though not for NSDMG).
Lack of expertise: This is the sort of thing one learns by doing. While there are a lot of model U.N. survivors out there, perhaps not many of them really feel comfortable designing an event of their own.
Playing time: NSDMG and Megagames are usually about 8 hours long. Many model U.N. conferences run for four days. That might be more than many people can schedule.
Location, location, location: Just finding a space large enough and with the proper setup to hold one of these events is tricky. NSDMG uses gaming conventions, which are usually held in convention centers or large hotel conference areas, while Megagames apparently use a variety of venues, ranging from schools to boats and more. High schools and colleges can often use classrooms and auditoriums. But for a general recreational interest, without a connection to a college or the budget to rent conference space, finding a site would be challenging.
Lack of familiarity: People aren't used to this sort of thing. And hanging around with a hundred strangers immersing themselves in a political simulation doesn't necessarily sound appealing. Megagame Makers have the advantage of having built up a cadre of loyal fans, and NSDMG benefits from staging events at gaming conventions with a high number of wargamers in attendance, who are a more receptive audience than the general public.
I'll revisit this issue again. A lot of these issues seem to be manageable, so long as a small group is willing to put in a fair amount of time taking care of the substantive and logistic issues. At the very least, it doesn't seem impossible to develop some kind of constituency for this sort of thing.