The clairvoyance artifact you can spawn in will automatically reveal what you would see with full vision (roughly a 7x7 area around you). You could also long distance teleport, which lets you teleport to any overmap tile which, obviously, maps that tile.
Otherwise, just save, reveal the entire overmap, then Alt+F4 to close the game without saving. When you load back in you’ll be back at your save point with the map obscured again.
