You can use any shader/node that provides color as a mask for populations. One approach that often works well is to create your new shader that you intend to use as a mask, let's say it's a Power Fractal, enable High and Low color so you'll be able to see it, then put it into the shader network as the last shader before the Planet object (in other words the output of this shader feeds directly into the Planet object, and all other shaders feed into its input). What this should do is completely replace your terrain colors with those of the Power Fractal noise function. What you can then do is visualize exactly how your noise function is going to apply to the terrain. The white areas will show your masked shader, the black areas will show whatever comes before it in the node network. So you can tweak your shader to create the shapes you want (this works with a Painted Shader or most any other color-generating shader too), and then disconnect it from the main network and feed it into the shader you want to mask and viola!
- Oshyan