It turns out that one can edit the world map pretty easily with SH3MissionEditor.
I did this by adding a "Naval Base" over a lighthouse, then named the naval base a lighthouse.
In the map this would probably cause problems - these lighthouses are now friendly bases which you can probably dock in
I would have to try these to see if the problems can be managed. The lighthouses done like this would be seen in every map zoom level, so it's tempting to see if the mod could be done like this.
Another problem is that naming the lighthouses garbles up the higher zoom levels with text that is too close to each other - "Brest" would be partly covered by the lighthouse names.