No way to play for Portugal (as in nation quest), but you can sail under a Portuguese flag. There are also no current settlements. MK had wanted to add a town somewhere on the continent, but I don't know what the status of that is.
But to add something like that is not trivial. Locations need to be added and defined in the scripts, models allocated, characters added in the game startup functions, worldmap/travel coordinates set up, worldmap model and locator tags updated...many things I'm probably forgetting at the moment. We did it once when adding Boddon's Landing, but I've since forgotten the many details, though I probably have notes about the process somewhere.