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 (also available in DAZ and HiveWire3D flavours) 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.