Thank you, all of the above works.  A PNG will keep an alpha channel, which means that if there's no info for the BG, it will be transparent.  In Photoshop, it's automatic as well.

I've found that the default lighting that you get when you boot Poser is exactly the light needed to make the pearlescence work.  While other lighting combos can do the trick, the more I fiddle with it the worse it gets.  This even works well with multiple figures and props.  I have to keep the scene in Smooth Shaded Preview, though otherwise the lighting falls apart mightily in other formats.  That's why I like Smooth Shading so much: you immediately get the best sense of the contours of the models without having to try out exotic lights.  This makes them powerfully useful as visual reference objects in greyscale, not at all useful as final renders if you want colour or to see eyeballs. 

I'm pretty sure I've looked at every screen and control Poser has to offer, but for whatever reason I completely blanked on the Preview Render window.  I wish I asked this question months or even years ago, I could have saved myself a bucket of trouble.  In gratitude, I promise to name my next child after you.  I doubt my wife will go for Red Phantom Hborre Twelvefield, but I do have some time to get her to see my side of things.  More time, now that I have this new trick!