Me too, Peter. Ive seen clear mouthpieces (acrylic). And there are mouthpiece visualizers which fit into the horn–basically full mouthpieces with the cup cut away.
The problem though, is we really need to learn this by feel, not by looking. Looking is helpful, I think, when trying to help someone else but we need to learn the feel, sensations and it has to become subconscious, second nature.