It's actually even simpler than that, thanks, Ogre.
The simple shape-merger can be used as the blender on its own to a surface layer with the cloud fractal as colour function.
I'm finding the cloud ring cut-offs are coming from bad cloud height/depth settings, rather than the contour shader. The closer to the centre of the planet you set the height - the more cut-off there is behind the planet. The contour shader method relies on the ring layers being constrained within it's own limits. This makes it a very tight squeeze.
I'm going to test with 4x larger contour spacings and offsets to see if it helps, the rings can then be wider and, hopefully, not cut-off...
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