Good stuff. Without knowing the situation (myself) couldn't a high late fee be the solution? The high late fee makes those renting cars very likely to return them on time to avoid the fee. It isn't clear if you have data that the high late fee is just a penalty that doesn't make the system perform better or not (to me reading this anyway). I agree, I think zipcar is an interesting innovation. It would seem to me real time (internet enabled communication) would help a great deal - notify of bottleneck, report cars needing service, tell user that car is late but these 5 nearby location have a car... Some of this is just trying to make the problem have a lessor negative impact.
I really like innovative ideas like zipcar especially that find solutions that are more efficient. Zipcars can reduce the waste of cars sitting around unused in millions of driveways.
Related: Zipcar Innovation (2008) - Traffic Congestion and a Non-Solution - Customer Focus and Internet Travel Search