What I was thinking was using FMRI to find out if they actually do remember the password (FMRI lie detection really only works with yes/no questions, AFAIK). If they don't know, then torture is a waste of time. If they do know, then you know torture may be fruitful.

FMRI uses indicators like pulse, heartrate, etc. to make a more or less estimate on the truthfulness. Torture can make these indicators useless. Torture is a very flawed method to extract informations. You can't be sure that the victim isn't telling lies or admits to crimes just to make the torture stop.