I am not sure that adding a county code would help. I just put one and one together.
You mentioned the fact that the OSM data from http://garmin.na1400.info/routable.php does not contain a country code, because there are several tiles for one country and several countries for one tile.
On my Nuvi I loaded the OSM data successfully. Thanks a lot for this wonderful service! On the Nuvi the destination addresses are limited to the currently selected country.
The OSM data on my Nuvi contains the map of Kenya, but Kenya is not available in the country list and so it is not possible to select an address in Kenya as a destination.
How does Garmin add new countries to the list of countries? I guessed it would work via the country code, but it is only a guess.
On my nuvi 205 the address search for cities and streets is limited to the currently selected country.
And since the OSM data from http://garmin.na1400.info/routable.php has no country code (as described above) it can not be selected as a destination for routing.
Is there a solution around that?
One idea as a suggestion:
Would the nuvi be searching the OSM data if everything would have the country code "world". This would make the search maybe slow, but at least the OSM data could be accessed.