EDIT on my above comments.....if just the "rotation" (ie. the spin in the up direction) is taken from the ArchiCAD Object, that will work. It's when there are rotations in 2 axis that the problems occur with axis order.
However.....another complication is.....if you add a chair, proxy it to a tree OBJ file, then spin the chair 15 degrees in ArchiCAD (spin around the up axis), then if you apply additional (non-up axis) rotations via the plugin proxy info sliders, you will get odd results, because there is already a 15 degree rotation in the (Octane) Y axis, and any ArchiCAD X axis rotation will be applied before the ArchiCAD supplied rotation.
By FAR the easiest, safest, simplest, least complicated way to do this is to rotate the proxy via the proxy info rotation sliders. Any additional rotation from the ArchiCAD Object is going to give unpredicable results (and a support headache for me

)
[EDIT: John's requirements of taking ALL the position and rotation info from the Object should be OK, because in that case the plugin proxy info rotation sliders will be disabled.]
Paul