I am looking for a way to create a custom weight mask using the Point Weight Component. The only option there is to use a sphere as a shape.
What kind of custom weight mask are you looking to do?
If you go into the component (hit enter when selected or zoom into it), you can see how the current sphere weight map is created in the shader (glsl1_pixel) using a distance and falloff function. The distance function is currently generated in the parexec1 to represent a unit sphere, so it calculates the distance form the point to the origin (0,0,0) and subtracts the radius 1 so that positive numbers indicate how far from the surface of the sphere it is. (Points are multiplied by the inverse transform so we can use a simpler distance function).
If you’re interested in a different shape, you can write a different distance function that returns the distance from a point to a cube, or cylinder for example.
Hope that helps. Let me know if you have any more questions.