You could make it so, that every wall/floor/object gets visible when the player can see it. Before that they are invisible. Now if the mini-map is a screenshot that refreshes every 10 seconds(or somthing like that) there is only black where the player hasn't been! But then you would have to assign an action to every wall and object... This is the only way that i can think about...

Hope that helped(i dont think it did)

~Phantom88~