What Buddhist materials describe how to increase the number of people with good Karma in a geographic area? In some of his books Lama Ole Nydahl claims that the prosperity of certain region is determined (among other things) by the Karma of people living there. That is, people in Switzerland (or Silicon Valley) have, on average, a better Karma than inhabitants of poor, war-torn countries.

Imagine that the latter want to improve the situation: Gradually (over several decades or even centuries) build up good Karma so that the life in those regions becomes better (turn Sierra Leone into an African Silicon Valley, for example). Also, there is plenty of evidence that economic measures alone often don't fight poverty effectively, so why not trying out a Buddhist approach to economic development?

I assume that there are some publicly available teachings on how the average Karma of a region can be improved. 

What are they and where can I find them?
Though not directly affecting karma, there is a variation of Metta Meditation mentioned in the Visuddhi Magga that increases universal love and brotherhood. Though the object is universal, its obvious that nearer vicinities would be felt more profound an effect.

If there is one thing in this world that can influence peoples' Karma, it is meditation. Karma is nothing but the result of aeons of conditioning of the five aggregates, and only with insight (pradnya) can they be positively affected.

As a practical experiment, just arrange a series of group meditation sittings among the people of the geographic area you want to improvise. After each sitting, just feel the metta vibrations in the air!