My Chrome extension (which I don't use anymore, but here it is: https://chrome.google.com/webstore/detail/cryptopass/hegbhhp...) uses similar technique, but instead of displaying this "PIN code" it uses different colors for master password input box text: the color changes as you type.
Saw the colors thing before. Interesting but I think I prefer the PIN because it's easier to check (you can't compare color shades on different screens), though I must say I haven't used the PIN method before (I have used the color method and found that it didn't really add much).
