Categorized symbology with transparency including raster basemap?

I’d like to include the background map in legend symbols, rendered under transparent fill.

For matters of better representation and easier orientation, it would be helpful if the symbols in the legend (right part in image below) had the underlying layer (the background map, cf. left part of the image) rendered below the current legend symbol.

If need be, I can just make a small cut-out of the background map with snipping tool or what have you, save that as a .png and somehow incorporate that in the legend symbol. It doesn’t matter, if it’s the same piece of background for every legend item symbol.

PS: I found out how to make the legend symbols have transparency, so that’s not a problem any more.