Remembered that some time ago, I was reading about the way to do corrective blendshapes from the HyperReal series from Alias Masterclass. Even though there are already lots of corrective blendshape solutions out there on the net (Not forgetting the powerful PSD solution scripted by Michael Comet), I kinda like the approach from the class. It's simple, and yet does the job well.
I even scripted the workflow out into a simple tool, but not sure whether am I allowed to share this. Let me email the relevant people to see if I can do so.
Would be adding an additional tab to the tool for setting up set-driven keys easily. Might add in the workflow using the poseSpaceReader to drive the the blendshapes.