I know this is an old post, but if you see this, I have to ask, why not just back up your driveway? You won't need regenerative braking when backing up uphill. Gravity does it for you.
Also, I'd like to point out that most people probably back up, oh, maybe a mile per year? At most, maybe a few? And at very slow speeds, too, so negligible kinetic energy. Does it really make any sense to complicate a design to try to recover this energy?
Next, yes, you can't program the Spark to stop at partial charge, but if you live at a high elevation, here's a makeshift solution. You can program a delayed charge time based on departure time. Just set the departure time for a few hours later than your planned departure time. When you're ready to leave, the car will be a few hours shy of a full charge. Not a perfect solution, but may work for you most of the time.
And last, I've noticed, now in a few places, mention that Teslas are always set up for "one-pedal" driving, with no control over regenerative braking from the brake pedal. Really? Is this still true? Just how is this set up? I find it hard to believe that a car with that many digits in the price would be so monumentally undrivable. To be really properly useful, you'd need to be able to get at least 0.9g of deceleration from regenerative braking (it's a performance car, maybe more, especially at speed with aerodynamic downforce). The response would need to be instant. Do you mean to tell me that if my foot slips off the "gas" in a Tesla, even for a fraction of a second, that I'd be plastered against the windshield?