I just found out yesterday that there is a namthar of Terton Migyur Dorje, the terton who revealed the Nam Cho treasures, and that there is an English translation. In fact there are three namthars, all written by Karma Chagme. The outer namthar has been published by Palri Publications in Nepal and is titled: "The All-Pervading Melodious Sound of Thunder: The Outer Liberation Story of Terton Migyur Dorje", by Karma Chagme, translated by Lopon Sonam Tsewang and Judith Amtzis with a foreword by HH Penor Rinpoche.
I'm trying to find out how to order copies now. However, the entire work has also been broken down into multiple pdf files and is available from the Tibetan Buddhist Resource Center.