Last time I used it, Maps did "Downloading Data..". whatever that means. But checking now and visiting an area I have never been before (USA) it doesn't seem to want to pull maps for it.
If a "Here Maps" client is possible (or anything else that has offline map caching in a way that''s as easy to use as Nokia Maps) then yes that is an answer.
And yes you could write a shim. The hardest part is reverse engineering all these libraries (libatlas.so, libmessagebus.so, npatlas.so, npatalsurlresolver.so and npmessagebus.so) and finding exactly which Gecko interfaces they use and if those interfaces need translating.