I am not sure how to replace the text with a word pin (as you type, like a password prompt). But if you are just talking about encryption of an existing string you could employ a simple ROT13 or any other simple encryptions (even a simple XOR encryption will do for most cases-i.e. not overly sensitive data).
I tried to run that lisp and I get a "string type error" meaning, it didnt work. Sorry.