I guess you've got a healthy sense of humor!
Now, I think you have more problem than just shade. Won't you have a bit of a drainage problem, unless the toilet is cracked at the curved part of the drain or near the bottom of the bowl? The toilet always holds water due to the water "seeking its own level" between the bowl and the drain curve. That means you could have some very soggy soil in your toilet . If it gets a lot of rain or if the toilet gets water from sprinklers, it could kill plants that root as deeply as the water level.
So, to follow a real theme, perhaps you could make a miniature pond or swamp garden---all nice and wet! There is a miniature papyrus that is really cool and some wiry, outer-space looking cotton sedges that would knock your socks off. Visit a store that specializes in ponds and ask about "marginals." Marginals are plants that grow in the sometimes dry, sometimes soupy-wet edges of ponds. They'd be great for a toilet with no drainage.