I guess car rental agencies don't have a policy of "walking" you to a comparable rental elsewhere the way hotels do? I've never experienced the no car problem, but I'm kinda surprised they don't have something in place for this. They're doing yield management, and if they screw up and have too few cars (or someone overstays, which can happen in a hotel as well), then they should be fixing that for you seems to me. But I'm not versed in the official policy, which I'm guessing states you are SOL....