so far as i know, there isn't such a thing as a place that will let a person borrow a bike just for leaving a set of keys as collateral. maybe with a credit card too? you could just bring them a set of any keys, how would they know? certainly not trust, this is san francisco. pedal rev might still, ye olde bikes might. on the off chance that either place did lend out bikes i'm sure they'd want more than just keys.