I only have a modicum of knowledge about posts, but I don't think the kinematics of your machine match anything that is standard in the g-post option file generator. Here is a good explanation of post-processing math for multi-axis, if you don't mind linear algebra:
http://linuxcnc.org/docs/stable/html/motion/5-axis-kinematics.html
The linuxcnc.org community does this kind of hobbyist robot/mill project, maybe they can suggest ideas.
