The textoverlay element has a property called "shaded-background".
It would be nice if that function could be extended so that you could control more  of its appearance:

COLOUR of the bar (including opacity)
opacity should probably regard only the bar and not the text

MARGIN between rows
allowing two rows of text to appear as separate bars
preferably as a percentage of the current bar height (which would be the sum of the font height and the vertical padding)

PADDING between bar edges and the text
preferably as a percentage of the current font height

Whether several shaded rows should always be equally wide, or if each shaded row should stop where the text ends.
To understand what this means, simply select this very paragraph and notice how the complete selection is not rectangular.