I don't think so. setPercentage(<float>) isn't about transparency, it's about how much (in terms of length) is visible of the polyline. But because it can't cut a line segment in half, it can only make full segments visible or invisible. That means that a Polyline that consists of a single line segment, will only be visible at 100%.
For transparency, there's actually the setTransparencyMode() method to enable it and the color attribute of a Polyline to set the desired value.