It can't be done, either manually or progamatically. This is because the color behind every slide master is white. If you set your background to 100% transparent, it will print as white.
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). In this way you would retain transparency.
Here's an example of how to export all slides' shapes to seperate PNG files. Note:
This uses a depreciated function,
namely Shape.Export
. This means
that while the function is still
available up to PowerPoint 2010, it
may be removed from PowerPoint VBA later.
Sub PrintShapesToPng()
Dim ap As Presentation: Set ap = ActivePresentation
Dim sl As slide
Dim shGroup As ShapeRange
For Each sl In ap.Slides
ActiveWindow.View.GotoSlide (sl.SlideIndex)
sl.Shapes.SelectAll
Set shGroup = ActiveWindow.Selection.ShapeRange
shGroup.Export ap.Path & "\Slide" & sl.SlideIndex & ".png", _
ppShapeFormatPNG, , , ppRelativeToSlide
Next
End Sub