Is there a way to make a spline around an object
and use it for where you want the brush to deform.
or have the spline create a mask based off its area of influence.
ZBrush doesn’t have a spline feature.
You might be able to get the results you’re looking for using ZAppLink and then painting grayscale in Photoshop. Once back in ZBrush you could mask from what you painted, then use that mask to control deformations and sculpting.
Another option would be to use Photoshop to make a 16-bit grayscale alpha that is the design you’re looking for. Import that into the Alpha palette in ZBrush and then make it into a stencil to sculpt through.