With some difficulty.
You can call this from map.lua:
Graphics.activateOverworldHud(WHUD_NONE) to disable the border and draw your own hud in its place. This will give you an 800x600 canvas to work with.
You can then prooobably also move the camera in lunalua's onDraw function to offset the viewport relative to the player... though I'm not 100% sure on that.