Maps should be able to show a specified KML file as an overlay/layer. From mw:Talk:Maps#KML:
Overlaying a KML layer would generally be done by the map library, (e.g. leaflet), and there would be no problems doing so with these tiles. Pnorman (talk) 23:38, 24 September 2015 (UTC)
Note that until T28059 is resolved, KML files have to stored as wikitext on template subpages. The actual file can be accessed using action=raw, and a page's related KML file can be found through Wikidata Property:P3096 (see Meta:KML files and d:Property talk:P3096 for further information).