If I'm not mistaken there was a short discussion about Mistral here recently. The other day I got my hands on some old issues of Herbert Spencer's one-of-a-kind Typographica magazine, and I found by far the best treatment of the font (written by Excoffon himself), in issue #12 of the Old Series. The article contains developmental sketches as well as a technical analysis - it's a true gem, and probably a goldmine for script designers.
It's from 1956, so if anybody can assure me that the copyright is passed, I can
hhp