I wonder how many scout troop in North America, or around the world, has gone reasonably light and cheap, carrying their own gear on multiple overnights.

I am not sure how to define reasonably light and cheap.
Depends on the size of scouts and the climate I suppose.

Perhaps as a simple squared function of height, like...
4'0" = 16 pounds pack weight, or even skin out weight
4'6" = 20 pounds ''
5'0" = 25 pounds ''
5'6" = 30 pounds ''
6'0" = 36 pounds ''

Here is a growth charts for boys, for the curious...
guidelines.http://pediatrics.about.com/library/growth_charts/nboystwo.htm

I bought the Field Book for Canadian Scouting the other day. I must say its a great book for $10 and I've heard some middle school and high school teachers actually use it for some course curriculum. It has alot of good stuff in there, including advice on buying stuff, but nothing that I recall about limiting or minimizing the amount of weight they carry. You would think that Scouts would have some reasonable guidelines. There's got to be something. It's pretty basic stuff. I think medical associations have already spoken out about books and book bags in the last few years, which have gotten way out of hand.