thanks for asking. This is a very interesting question.

It might be possible, but probably only by hardwiring this 
functionality globally to a button as any UI that is open - or could 
open - is covered by the screensaver / login / password window combo. 
I don't know of any workaround to this, either.

You may want to try this Applescript snippet with your own custom 
action:

tell application "System Events" to keystroke "mySecretPassword"

(all in one line)

Best regards, 
Felix