there is an update for LabCurves to have a curve for adaptive (or absolute) saturation adjustments. It is an external 16 bit program which can be semmlessly used from gimp with a python script.
More information here: http://www.mm-log.com/blog/2010-07-09/adaptive-saturation-curve-labcurves
Unless otherwise noted, the rights for the individual plugins are with their respective authors. Logo image by Jakub Steiner.