At this time Blender contains decomposition nodes only for HSV and RGB, but not for HSL color model. Because I find HSL color model very useful for color manipulations, especially the Lightness value, I created both Separate HSL and Combine HSL nodes, which enables to modify each part of this color model.
The both nodes are created using existing Blender nodes, not by OSL script. So, you can use my nodes for GPU rendering. I didn't use OSL, because it doesn't work with GPU rendering.