The person you are looking for is most likely Larry Korn, who worked on Fukuoka-san's farm, translated his
books, and has been a major figure in spreading the One-Straw Revolution in the West. Larry makes himself available through
his website, where you can also find an email address.
I would add that in all of Fukuoka's books, you see him deeply questioning mainstream science- look for phrases like "discriminating knowledge". Because Fukuoka had esentially achieved enlightenment, he was beyond a scientific/material understanding of farming and nature, and that holistic understanding is the absolute basis of his work in natural farming. So while he did record yields, he was most definitely NOT operating an agricultural research station. Think of it this way- he wasn't conducting an experiment. He wasn't doing double-blind, controlled trials of his methods. He so deeply understood the correct way to work with nature that he just farmed, and you can believe him or not when you read the results he reports.
This is one of the difficulties with Natural Farming- replicability. There are many people practicing Fukuoka Farming, but it's not a method that you can just call up your Extension Agent and get advice on.