L’Art de la flûte traversière
The treatise's first reference to improvisation is the explanation of the ornaments with a short description and later a table in musical notation.
It is here in full:
There are also 12 Caprices (capriccios). Sadly the author does not say anything about how to improvise or compose them, but the examples give ample ideas with many tempo changes and wild figuration.
There is also an example of quarter tones inspired by Ancient Greek sources.