I've gone as an advisor on the overnight, I'm also a WS MB counselor. You're right, they don't let them take very much. In fact, they won't let them take many of the Scout Essentials that they are taught to have with them whenever they are on a hike. They won't let them take things like any type of cord, wire, rope, (makes it too easy to build shelters.) They can't bring firestarting material, food, shelter...

I think they basically were allowed the clothes on their back, a poncho, water bottle, flashlight, and in our case they were allowed (required) insect repellant. (Risk of EEE from mosquitos and Lyme Disease from ticks is pretty high around us. Deet's a way of life.)

Now, we wouldn't suggest they ever go out like that, but, in this scenario, they are really encouraging improvising. I think the guys all come back pretty proud of themselves when they realize how well they have done with so little to work with. I included some pictures of shelters some of them built this summer in an earlier post in July:
http://forums.equipped.org/ubbthreads.php?ubb=showflat&Main=9256&Number=101036#Post101036