I'm having trouble finding this useful. I don't see what I can do differently with an xpm and png overlay than I can just do with a single xpm. Even if you want to use advanced opacity features of png, you can still just pop it over the xpm in gimp and merge the layers for the exact same final image.
The only difference I can see is if you want portions of your image to have an independent palette from gtk if you are grabbing those colors in your theme.
A themerc option where we can make the png overlay display on a mouseover or something might be neat.
I've recently started looking into theme creation for xfwm - so I'm just looking for inspiration and trying to find the limits of what can and cannot be done.