If you found the GO #100 (the Lieber Code) of interest you will like the GO #20 passed by Winfield Scott in Mexico. You will see a direct line of precedent from one to the other. Evolutionary in nature. Scott laid the foundations for the law of war, this was nearly revolutionary and very American.

The best reference for this GO that I found is available via JSTOR. I've found no fully open source.

http://links.jstor.org/sici?sici=016...3E2.0.CO%3B2-F

as well as Ralph H Gabriels "American Experience with Military Government" American History Review XLIX, July 1944, 634-7. again JSTOR.

If I can find a hard copy I will try to scan it and send it in.