As a Portuguese physio I new about Mézieres many years ago.
In fact Mézieres, RPG (Souchard) and Chaines Musculaires (Busquet) are not the same but all come from Mézieres. The most used are RPG (Souchard) and CM (Busquet) and both are Ostheopats who work togheter from many years.
That's one reason why do not exists so many studies as other areas of medicine. Never the less, those méthodes and téchnics are not "quackeries", they work when you now how, when and with who. Like surgeons used to say "we prove our skills with the good cases". So, even you do not no why it works, you can use it because it works. And remember, in physiotherapy (and medicine, even in pharmacology - remember Vioxx and other stories) almost of we done everyday are not proven...

Best regards