Thanks to millighost I now know why my normals aren't working: the outputs of a Normal Map brick are the normals in camera-space after being applied to the object. I was incorrectly assuming that they were still in tangent-space. This makes the maths a bit more awkward, but at least I'm now on the right track with the normals!

 

A few further thoughts

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Overlay Mask: Two sliders seem a good idea to make any mask image more flexible. Contrast + Brightness perhaps? Or Black Level and White Level?

Reflection: Perhaps only one Environment Colour Map/Trace/Mix brick network should be used? The base and overlay will be reflecting exactly the same thing, and it's only the reflection strength that is likely to differ.

Opacity: The overlay mask itself is the opacity of the overlay!

Refraction: Since the overlay is assumed to be a thin layer applied on top  the base, the effect of overlay refraction depends on overlay thickness, which does not exist (except perhaps if we use bump/displacement). Could add a 0-100% slider to let the user set the value?

You know those kid's balloon with coloured designs on them? And when you look through the balloon at the back of the design it's just a flat opaque colour? Can't see a way to do that.

 

Other reminders to self:

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I noticed when I was doing my tileable mail textures that normal maps didn't tile in DS3. They can easily be made to tile correctly using Shader Mixer.

If you combine two tileable Image Map bricks with a Mix brick, but set different tiling values (prime numbers) then the pattern repeat is increased to the product of the two numbers. Might be useful for large plain surfaces?

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).