narrow color bar using display properties

[Eric Abrahamsen]

I've got my cursor-type set to a colored bar, and looking at it today I
thought it would be very nice if I could create a similar effect with
code: presumably using text properties (I'm guessing the display
property) to lay a bar of color (either background color or a
transparent foreground color) over existing text, only a couple of
pixels wide and aligned left, without otherwise disturbing the placement
of the text beneath.

Right now the most promising approach seems to be the image display
property (presumably I could use the :data specification just to draw a
block of colored pixels), except that if I understand it correctly that
will replace the underlying text, not add to it.

Does anyone have advice on achieving this?

Thanks,
Eric