Normalized Transform Node
So this idea came from a variety of sources, the "A Hybrid Approach to Facial Rigging" paper from Disney, our facial rigging lead at Boulder Paul Aichele, and trying to consolidate a range of hacky solution I've used in different situations.
There's definitely room for improvement in animator-rig interaction.
Of course having a different manipulator for the transform and normalTransform nodes is a major problem but I'm working on finding a user-friendly way to fix this.
This custom transform can't achieve the behavior required for the inner brow twist outlined in "The Art Of Moving Points", which would require the rigger to be able to set different translate and rotate pivots for the manipulators, and have the translate get applied in the post-rotate space. But something like that may require a robust riveting system so that we get the position and orientation of the control and the actual transform attributes don't affect the matrix of the control.
Learning about this part of the API has definitely taught me some great lessons about rig design, and the internals of Maya.
At the moment I don't intend to release the code for this plugin.