Zipcar's hourly pricing is so low because the rental for these extra 'waiting' hours is built in. If you are driving at 60 miles per hour and the car gives 20 miles per gallon, the cost of driving for one hour is over $10 at current gas prices. Zipcar's hourly rentals don't cost much more than $10.
Your idea of using cars others have parked locally is a good one, although I wonder how many people rent zipcars for 7-10 hours and park it for 5-8. It would be almost certainly cheaper to rent by the day from Enterprise et al if you did that.
It often is cheaper to rent by the day from companies like Budget, but zipcar is incredibly convenient. In DC, I have 5 locations within 4 blocks. The major car rental companies are all 1 cab ride away.
I really like your idea but if Zipcar let you rent someone else's parked car, and you were late, they would almost certainly lose the stranded customer.
I know how much people hate waiting for a ride because my startup RideCell worked on real-time carpooling for some time before we pivoted to fleet automation. Even now, one of the main problems our product solves is reducing wait time for transport/service vehicles by auto-dispatching tasks to the nearest one, routing them intelligently, and making the wait more bearable by letting people track their transport/service vehicles (Think Uber for everything).
Of course, the fact that this is a hard problem makes it even more interesting :)
Your idea of using cars others have parked locally is a good one, although I wonder how many people rent zipcars for 7-10 hours and park it for 5-8. It would be almost certainly cheaper to rent by the day from Enterprise et al if you did that.