The show picture command. You're going to use a black image the size of the screen with the center cut out.
It'll stay in the same position on screen as the player moves around on the map.
This only works while the player is also centered. If you get to close to the map's edge the character will start moving away from the cutout and the center of the screen. The simple solution is to pad the edge of your map with 7 or 8 spacer tiles so this isn't possible.