I wanted to note here that I have gotten useful results with the Firefly methods revealed upthread.  The images I posted above showed a problem when shadows were used on the two-sided cloth in a scene with IBL lighting.  As illustrated in the images I posted (although not explained properly at the time), the two-sided effect is broken wherever a shadow is cast upon the two-sided item.  The fix is to set all shadow-casting lights' shadow intensity below 1.0.  At shadow strength 0.95, the effect works as desired, but at strength 1.0 the whole thing fails as I've noted.

I have not tested anything with IDL and would guess that it would fail, as smarter users than I have already noted, up there in 2022.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.