Do you mean like a reverse watermark?
With a watermark it's placed on top of the original image.
A reverse watermark would place it behind the original image?
I've been looking for something that does that in batch as well. We have some generic items that use keywords (names, places, etc) that could be placed on many different substrates and in order to display them appropriately online each keyword needs to be shown (instead of a "your name here!" place holder).
We need to create thousands of images for each type of substrate, so each keyword is shown. This is very time consuming (I have my wife do it). It would be much more efficient to create the keyword image with a transparent background and then have a program run through each keyword image and place the substrate image behind it. OR place each keyword image on top of the substrate image.
I've got several opensource image manipulation programs that perform many functions in batch and have literally saved me months, if not years, worth of work by performing tasks that normally would have taken a month to do manually in a matter of minutes. Unfortunately I have yet to find one that can do layering in batch like this.
Then my wife would only have to create a master list of keyword images and a program would do the rest.