)Knowing the angle of each section, From the intersection of the mediane ( I don't know if it is the same word like in french, but roughly speaking, the line that starts from center of circle and divides the section angle in two halfs ) and the arc of section, we draw a perpendicular line to the tangent of the circle that include that point of intersection. The length of that line could be measured since we know the coordinates of the square that wraps the circle, and the length of the label. Well, then we display horizontally the label at the end of that line. We could maybe also add a new line parallel to the horizontal axis and tha starts from the end of the first line, this assure us that all labels are parallel to each other...
I hope I have been understood. If no, Tell me were to send a pie example image.
Regards
Morpheus