So I've finally found it! I wanted to share because this might be useful for others. The following site -
http://refuge-huguenot.ish-lyon.cnrs.fr/index.php - has a searchable database of the charity or assistance records from Switzerland for the Huguenot refugees that settled or passed through there in the late 17th century. Most of the refugees who settled in Germany passed through Switzerland.
I knew Pierre Chastain (my earliest known ancestor) came from the Dauphiné province in France and settled in Louisendendorf, Germany by the summer of 1687. I also knew he was a doctor.
I searched the database and found 3 records for a Pierre Chastain. He received monetary assistance once in Neuchatel, Switzerland in November of 1686. Then again two days later in Neuveville, Switzerland. Then finally, in February of 1687, several months before he was known to be in Germany, he received assistance in Schaffhausen, Switzerland. If you are familiar with the Huguenot Trail, this follows that path exactly into Germany with each stop getting closer to the German border. (
http://www.surlespasdeshuguenots.eu/carte-sentier-mai-2013.pdf). The trail passes through all three of these and then on up into the Hesse Province of Germany where Pierre settled.
Now, onto the big discovery. Along with the assistance records stating the name, they also state the province of origin of the refugee. For this Pierre it was Dauphiné, which is a match with my earliest known ancestor. It also states his occupation as a doctor, which is also a match with my earliest known ancestor. And the fact that he was in Swiss towns on the way to Germany in the exact years and months that I would expect him to be, I think chances are very high that this is the same Pierre Chastain in my family tree. So, the big discovery? The records mention the town of origin for the refugee, which I had not been able to discover until now. The town is Vesc, France. A very small town once in the Dauphiné Province and now in the Department of Drome.
I now know where in France my family originally came from, and so I can try to pick up the trail there. Even if I don't find anything more, I'm very pleased with the discoveries I've made. I wanted to share this story and the link to the charity registers in case it's useful to others. Cheers!