This is extractable from the OSM Db and I know that User:Dispenser did
a similar thing to check the region info on individual coordinates.
Ideally, this information might be on Wikidata,
https://www.wikidata.org/wiki/Property:P1332 Co-ordinates of northernmost
post
https://www.wikidata.org/wiki/Property:P1333 Co-ordinates of southernmost
post
https://www.wikidata.org/wiki/Property:P1334 Co-ordinates of easternmost
post
https://www.wikidata.org/wiki/Property:P1335 Co-ordinates of westernmost
post
You'll have to be really careful here. Some countries cross the
180/-180 degrees line. More than you might think if you include
overseas territories!
Daniel