I think I have come up with a solution that I'll share here:
In addition to the window vector above, I am taking the point that the mouse pointer intersects with and the center of the bounding box, and making a vector using those two points. With that vector, I can figure out the angle between the window vector.
Code:
winVec = viz.MainWindow.screenToWorld([0,5,0.5]).dir
objAngle = [bbCenter[i] - objPoint[i] for i in range(len(objPoint))]
viewAngle = Vector.cross(Vector.norm(objAngle), Vector.norm(winVec))