"But, there are simple, either free or very cheap, stand-alone utilities that will create double-sided meshes for you from an original .obj. [...] I used to have one, but it's on an old, dead, hard-drive at the moment."

You're probably thinking of Objaction Twoface, by Maz, who appears to be no longer around. His Mover and Scaler are popular downloads which have been preserved elsewhere, but as 3dcheapskate notes, double-sided zero-thickness meshes became persona non grata with the advent of Firefly and I don't think anybody's bothered mirroring it. I can reunite you with it if you really want. :)

Ah, you're right! I have it, somewhere, stored safely away in the Poser-bin-of-forgotten-utilities-that-no-Poser-user-can-be-without... :) Since you named it, I will now be able to find it. Poser/firefly hates double-sided faces. With good reason, at least at the time. But, there are ways and then there are ways to make "double-sided" faces. For instance, in some older Poser assets, I noticed that they're made traditionally, with true duplicate meshes with reversed normals joined to the original mesh, much like a "thicken" algorithm will simply make brand new geometry and merge it with the existing geometry. ie: A single plane with one face and four verts becomes a box with six planes and eight verts. The key in algorithms that do this is the separation distance. As I understand it, Firefly has issues with faces with very small separation distances when calculating certain effects. So, doubling+ the geometry of a prop/figure in order to get some kind of material effect, that can be somewhat faked with other means, just isn't very efficient and is probably not really wanted.

But, hey, if you just gotta have that one material effect, it's an option for "gotta have it now" users, I suppose.

Now, I'm off to split some verts and torture Firefly with displacement maps slapped on them... :) Maybe I should make them Fifty-Shades of Grey, for added spite? :D

I thought I knew what "double-sided" meant, but It's a lot more complicated than I thought. :D     (I remember it was one the first things I ever posted about, back around 2009 - and EnglishBob gave me the lowdown. But I can't find that thread now)     As it turns out I don't even really know what I mean myself when I used the phrase !  :oS I do recall that using displacement was a possible workaround for the two opposite-facing-faces-with-identical-vertices (I'm trying to avoid saying 'double-sided'!) problem.

Extruding (i.e. turning a zero-thickness square plane into a flattened cube) wouldn't really work if the plane was transmapped would it? I'm thinking leaves, etc. The four 'sides' of the extruded flattened cuboid would need to be transparent, so it'd be just as if you had two opposite-facing-faces-slightly-separated.

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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.