First I do not know anything of python and I do not mean to waste your time. This is just a suggestion of a possible solution using Poser.

Is it not possible to attach a prop to an untransformed "standard hand" to the position you want using python, apply the necessary scale and translation/rotation changes to the figure's IK and then grab the world coordinate position of the prop again so you can calculate the vector between it and the camera. If you want a vector for the direction the finger is pointing you should be able to get that from the relevent IK section or by adding a second prop along the original IK section.

This is based on pure spectulation,  I hope it will provoke a more useful comment from someone who knows about Posers Python API.