I thought about a key press (like keys 0-9) to act like slots. That would definitely work, I'm just not sure it's a perfect solution. It'd work great if you switch levels or areas that use a solid color to represent different things (a forest, or a mountain, like Kirby's Dreamland). But what if you wanted the sky and the ground to be separate graphics? In the picture you posted, GBE would take the first row 8x8 tiles underneath the HUD and make them the same tiles as the ground. I think I have a decent solution, but I haven't exactly tested it out yet.