Don't think it is possible, but once paired it shouln't ask for it again. And my understanding is that most (if not all) BT mice have passcode of 0000. BTW maybe there is some hacking through bluez config files, never checked. And if you want to pair your mouse with your GF's phone without her know that, I would reccomend to choose moment very carefully, otherwise you may end up screaming in pain :P