Check out the new {{Attached KML}} template on en.wp Example use: http://en.wikipedia.org/wiki/Navajo_Nation
The WikiMiniAtlas (blue ball next to the coordinate link) already supports display of such vector overlays.
The discussion around the template can be found here: http://en.wikipedia.org/wiki/Wikipedia_talk:WikiProject_Geographical_coordin...
I am about to run a bot that would attach KML county outlines to all US county articles. A similar bot is being developed by User:The Anome for al US highways.