The Planet Surface Shader is *extremely* limited and experimental right now and frankly I don't suggest using it for anything beyond basic planet coloring and contours. It is definitely not intended as a universal, generic way to start the modeling of a realistic planet, and as such it is not very tweakable or easy to add onto. Most changes to it simply cause it to stop working properly, including adding displacement
In any case I wouldn't fiddle about with "get altitude", etc. for confining displacement. You should be able to use a Distribution Shader or Surface Layer as a Blend Shader on any displaced surface and thus control its area of effect by altitude, slope, etc.
- Oshyan