I've just taken a brief first look and I was pleasantly surprised. Even comparing SDK's Freehand beats Illustrator.
(I've looked at Illustrators SDK and it's NOT that easy to understand.)
FH XDK seem well documented and a couple of things caught my eye.
Import and Export seem to be possible through the XDK which could mean that a custom exporter (PDF) might be possible to create.
Another interesting bit I found was a mension of: Monitor Calibration.
Other that these I think the XDK provides great information on HOW Freehand actually works behind the scene if the need for creating a "clone" ever arises.
Will look into it some more...