how about a flip-flop hub? that will give you two gears, make the smaller one a freewheel use it to climb and the freewheel will allow you to coast down the hill, swap back to a bigger gear for the flats. i would run as wide a range as possible. maybe run something like a 16 or 17 fixed and then a 20 or 22 freewheel from a 29er singlespeed if you have enough dropout space to accommodate it