In this post Macro to do Slddrw to PNG without background?. There is a macro in the makeTransparent.zip file. The macro adds a image to the drawing... it should be in the background. I do not recall all details that I was working with. But it might give you a clue if this is what you are looking for.
Please note that the referenced post was targeting something completely different. I just recalled that I used an image in that process to achieve a result....
It is also a rather old post. There might be new API features to use for SW16 etc.