How to apply Corrective Smooth as Shapekey?

So, Blender has a Corrective Smooth modifier, which is nice. But I’m building a character for realtime, so I can’t use it directly. But I figured I’d take advantage of it by using it to create corrective shapekeys. It does have an “Apply as Shapekey” button after all, and it will be a good basis for manual tweaking later.

However, it doesn’t work.
For Corrective Smooth to do its thing it needs an Armature above it in the stack. However, with the Armature modifier there, it fails with an “Applied modifier was not first, the result may not be as expected” message.

So basically, how on earth can you apply Corrective Smooth as a Shapekey? Is it just a completely useless button that can’t ever work?