So I've been fighting with the Firaxis code for a few days now trying to get an overlay image to work for NoNoYesNo's request, with no great success. The way the map view is rendered seems to preclude me from drawing anything over it without it flickering severely or severely degrading framerate (and thus, usability).
So, I'm going with a simpler solution, which will just open a new window that you'll be able to set an image on, and that new window will be transparent. Drawback will be that you'll probably have to manually size the window yourself to match the underlying map. But with a customizable zoom control on the map, at least you'll be able to see the whole thing at once and compare it to the overlayed transparent image.
I've attached a screenshot of how it looked in an earlier build. It's one of those gigantic 5400x2700 Blue Marble images overlayed onto a 128x80 blank map.