Why can’t I add custom properties of any kind to bpy.types.SpaceView3D?
This page gives an example of what I am trying to do, but no resolution to the problem. That article is from 2010. Has anything changed? Is there an alternative way to work with this or can SpaceView3D properties only be defined in C?
Spaces are one of the types you can’t add properties to, there are actually much more types that don’t support custom props than those which do. You need to use Object, Scene or the like. If it’s not necessary to store the prop, you can also use a python class with static variables.