I've managed to reduce the blotchiness a lot -- I tweaked the Fresnel Spots when I realised it was the angled areas, such as the sides of the arms and head -- with hair, I was seeing the much smoother face area only. But you can still see a little of it. It may be that the Blinn node plugged into Alternate Specularity is having an effect too.

What I did was darken the spot colour, and shift it towards yellow (anti-Blue). That's made a big difference. No texturemap, I'm using a Simple Color node instead.

I wonder if average skin brightness is significant? Has anyone seen similar effects with African skin tones?