In this way you would retain transparency. The best you could do is design your slide with all the stuff you want, group everything you want to appear in the transparent image and then right-click/save as picture/.PNG (or you could do that with a macro as well).
If you set your background to 100% transparent, it will print as white. This is because the color behind every slide master is white. It can't be done, either manually or progamatically.
