I went over all the threads you posted... Waaay beyond my understanding of some of the intricacies of Poser's Material Room and Firefly Renderer. :)

But, there are simple, either free or very cheap, stand-alone utilities that will create double-sided meshes for you from an original .obj. The simpler the object, the better, of course. I used to have one, but it's on an old, dead, hard-drive at the moment. (There's also a stand-alone utility specifically for Poser that will make double-sided meshes single-sided, I think. I don't know if it's reversible.)

That doesn't address the issue of taking a Material Room hammer and beating the crap out of Firefly in order to make it do what you want, but a quick Google around might help someone who wants a "right-now" solution.