OK, folks, by popular demand, I've located a diagram of Marc Perrone's 3-row layout. I remembered seeing this on a French trad music website that has since disappeared, so I delved into the internet archives and pulled this picture from beyond the grave.
Enjoy. Also: If Laurent wishes, I will gladly remove this image from the blog; thank you for the valuable information that was contained on trad.org! It is my understanding that this layout is a standard that can be ordered from Castagnari.