I came across an interesting solution to this while paying at a restaurant in Ecuador: they used an Android device which randomized the position of the numbers of the on-screen keypad before each transaction. The original intent is to make it much harder for onlookers to guess your pin based on finger movements. This could however apply equally well to the usecase of the article. It is a bit of a usability trade-off…
There's a potential secondary usability trade-off. I only know my PIN by the pattern it makes on the keys. I don't actually have the numbers memorized, just the positional order on a standard keypad layout. So if the numbers are randomized, I would have a really hard time.
That's sort of a feature, because it forces you to adopt a strategy that prevents this type of exploit.