How it works is, anyone can go out and scout spots for it, and it pretty much only gets decided at the last minute. So nobody really knows too far in advance, because there is no selected spot. This helps weed out the bad, so I guess it is sort of selective, but I think in a good way, since most of the people that it weeds out are those who aren't willing to take the chance to go to the middle of nowhere on short notice (+drainbows)
I will say, it's fucking awesome and I haven't heard anyone who's gone say they didn't have a good time. Just head for the northwest around the time of it, you can count on the coordinates getting posted a couple days before.