No letters, but in theory you'd input a hero's name and see if it equals whatever your password should be. Of course this is assuming you have the Input Hero Name patch that's floating around out there (which still doesn't fully repair the function, so it's not that useful).

Key Input Processing is possible, but it still doesn't check for letter keys, and you'd have to have a separate variable for each key you put in. It also doesn't display the key you press directly (unlike Input Number) and isn't as hassle-free for checking the input.