Does anyone know how to get a 3D mouse cursor working?
You can use Screen.lockCursor or Screen.showCursor to hide the normal mouse pointer and then use the mouse data from the Input class to implement any type of mouse behavior, using some texture or model to represent the pointer. Obviously, a 3D pointer needs one degree of freedom more than an actual mouse device provides, so for one of the axis you will need use a control key together with the mouse movement.
I can tell you from experience, though, that 3D pointers without 3D controls are extremely clumsy to use.