As to that, this is what I've got so far:
Step 1: Export Character to Blender with Json and FbxKey: (Works now)
Step 2: Improve character!
Step 3: Export character FBX, write back changes and relative paths to the Json data, resolve potential naming conflicts with CC3.
(Still working on the Blender side of this part, for now I'm just overwriting the fbx from the import.)Step 4: Import and reconstruct materials in CC3 using Python scripts / plugins: (Notice the new menu)

Something like this:
Obviously Skingen data is lost, the complete skin would need to be saved out and reapplied to keep that data.
Also any hide body mesh data and mesh conform settings would also be lost on the clothing and accessories, so all of this would need to be taken account in the workflow.
In principle, it works. I need to get Blender exporting sensible data before I can release anything.