I don't think it is that good. It seems very easy for the bong or what ever else you are hiding to slid under the plug so that the water will not shut off. Resulting in someone lifting to take a look. Keep in mind, when you flush, that plug comes up. If it happens to get wedged underneath that while it is up and your dad is taking a shit, your screwed.

I would never consider something as unreliable as the crapper to be a good hiding spot.

Good idea, but that isn't original and I'm sure if your parents suspect something that will be one of the places they look.