Bosh! Thirty minutes of fiddling just now and I'd figured it out in principle. Using the alititude offset option and some pretty high depth settings I've got the start of something.
Figured I'd attach a jpeg of where I'm at so you can all see how it's achieved.
"Squish Northern Hemisphere" and "Squish Southern Hemisphere" are distribution shaders to push the Northern and Southern hemispheres towards the equator. Only thing is I started with 0,0,0 at the top of the planet, so I'll need to rearrange my scene a little. Still, the principle works on clouds... atmospheres, not so much as their altitude can't be modulated, but a fake atmo could easily be made as a cloud.
Ignore the node saying Transform Planet Back Up. That's not doing particularly much right now.