While reading the PSD specification, I saw that GIMP already has code to support importing several types of Adjustment Layers.
I did a quick test with connecting the Threshold, Posterize, and Invert code to our PSD plug-in, and it seemed to work well!
Hue/Saturation, Levels, and Curves should also be easy enough to add support for, since we already support PSD presets for those filters.