I think you want to use the "Lock Axis" ude to lock the X or Y axis you will also need to select the plane, usually the XY plane. This should force the rotary motion.
I tried to do the same thing. I obtain the C coordinates in the post, but they are not correctly connected with the moving origin and the ISV show a totally different toolpath.
Ad example if I program a zig zag pattern (floor wall), using the Ude the toolpath showed by ISV is similar to a spiral and the surface is not totally milled.
I think the problem is in the post, and I don't know what I need to modify.