Printing PictureFrame with Transparent Background

I’ve saved out a PNG of an exploded view of a Rhino model and I’m trying to use it in a PictureFrame dropped into a drawing border. Usually I use Make2D to produce a line drawing of the exploded view, but in this case the shaded view is much easier to understand.

When I come to print the drawing, if I set the output to raster, everything works fine, except (as usual) the whole thing looks a bit fuzzy because it’s, well, raster. If I print the same thing to vector output, the PictureFrame displays fine except that the background of the image shows as black instead of white. Is this is bug and more importantly, is there a workaround?

Rhino V5, latest SR.