That's exactly what I was thinking. If they ask for a fingerprint to unlock the phone just ask them "which one?". If the device will fail back to a passcode after a few failures then you just have to hope they don't get lucky. Maybe pick a finger for unlocking the phone that's not obvious (i.e. not index or thumb) or "misunderstand" which one they asked for if they get lucky and use a different one instead. Oops!