Thanks for the correction English Bob (you weren't kidding on your site when you said you can be found hovering at most poser-related forums).
I'd noticed that some .objs intended for Poser have the normals explicitly defined and some don't (should have realised at that point!) - I'd actually scribbled down a query "f 1,2,3,4 ≠ f 4,3,2,1 ?", so I think the fact was there in the back of my head.

On the subject of single/double sided faces - I've noticed with dynamic cloth simulations that I often end up with faces from the inside of the cloth towards the camera. These, of course, are not rendered (a double-sided mesh would render correctly).
I suppose that if you created a mesh with all normals pointing outwards, then duplicated it, inverted the normals, merged duplicate vertices and exported an .obj then, when rendered in Poser/DS, it should appear as if it was a double-sided mesh? Is this the a sensible way to get over the problem?

Regarding the .pp2/.cr2, I'd already come across Kuroyume's unofficial file CR2 format spec (.pp2, etc seem to be a subset of this?). Although perhaps not quite up-to-date it seems to be a very good explanation.
Guess it's time to consider the .obj and .pp2 files together., and try adding it as a prop rather than importing it as an .obj

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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.