There is no way you can change ECU programing and I don't think anybody hacked Prius either, but what you could do is to develop some simple ways to help the car get better FE. I don't live anywhere near hills, so don't have much experience but probably gaining higher speed before up the hill climb and bleeding some speed as the climb continues up should help your FE. Don't use breaks too much, if the hill is long and steep enough to overspeed use B mode instead and it's better to accelerate briskly but shorter, than slower and for much longer. Another thing I noticed was that you get much stronger electric boost if the battery is fully charged, than when it's around 3/4 or bellow, so it's actually good to have battery fully charged at all times. And probably most important: keep accelerator pressed down only enough to sustain your speed, sometimes very little change in accelerator postion and speed makes for huge changes in FE. I could never get used to the fact that by trailling throttle at 43 miles per hour I get 20 MPG and by just barely slowing down to 42 MPH I can get 60 MPG or full electric mode, it is similar at higher speeds.