I guess that the answer on the Poser side may lie in one or more of the following (from the Poser 6 PoserPython Methods Manual). But the 'explanations' really don't help me here. I guess I should just try printing all of these out in step (10) to see if any of them match the values I'm getting from DAZ Studio?

actor.AlignmentRotationXYZ() - Get a tuple comprising the ordered rotation alignment for this actor. (order is X, Y, Z)

actor.LocalQuaternion() - Get a tuple comprising the quaternion local rotation for this actor.

actor.Orientation() - Get the orientation of the actor’s coordinate system (x,y,z ordered rotation).

actor.WorldQuaternion() - Get a tuple comprising the quaternion world rotation for this actor.

(I never learnt about quaternions - I have the vague impression that they define rotations by specifying a vector/axis and a rotation about that axis ?)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident. And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU or something else.
*soon only to be available in two flavours -  DAZ and Renderosity