it would be perfect for a camera, but the visibility would be a problem with it that low. Normally cameras are mounted as high as possible point down so that you get the best sense of perspective. It's easier to judge approach speed and distance when an object moves across the field of view, rather than just getting larger as it approaches the camera lens. I'm reminded of a sequence in Space Balls when the camera 'zooms' (it actually dollies) into and hits Dark Helmet. It's really hard to tell exactly when the lens was going to hit him. I'm afraid that's what might be the case with such a low mounted camera. You could certainly see kids running across the back of your car, and any stuff just sitting there, but it wouldn't help when parallel parking.
parking sensors might work, but i think they like to put them in the corners, again, to help when you angle in while parallel parking.
this
emoticon guy might be a bit too big, but that could be pretty funny.