It's possible to use Brushify with Spherical shaped landscapes.
Here's an example with Voxel Plugins Spherical World:
The process is actually very simple:
At the heart of Brushify is a calculation for derviving a Slope Mask. This is called MF_Explicit normals. By default this is setup to work for a Heightfield/Flat landscape. However it's possible to alter the explicit Normals so they work for a Sphere. Go to your 'Content\Brushify\Materials\Landscape\Functions\General\MF_ExplicitNormals
Delete what's inside and paste this instead:
Save and compile (it will take a while as everything must be recompiled for this change because it's so integral to the shader). After that add the MI_Landscape material instance to your spherical landscape and assign layer infos as usual.
The image below is the spherical Explicit Normals applied to a rock geometry. As you can see the areas perpendicular to the centre are White (e.g. flat areas in relation to the planet core), while black areas are the Slopes.