Are you sure the injectors totally cut off all fuel to the cylinders when coasting down a hill in gear?? I could see them reducing the fuel (perhaps even adjust the timing too), but not cutting it all off.
Not that I want to risk it with my car and my life, but while coasting in gear, if you turned the engine off (one click back of the key) that would shut the spark off. If there was no extra engine braking felt after that, then I guess there is no fuel in the cylinder and thus no injector spraying.
(of coures this would only apply to a manual trans).
Now when I was back in high school ( a long time ago), I was coasting down a hill and turned the engine off while it was in gear. Since my truck had a carb (and other low tech features), I immediately felt the extra engine braking, but I also experienced the biggest back fire of my life when I flipped the key back on. All that fuel that was getting sucked into the motor and then the exhaust did not get a chance to burn until I hit the key!!! Wow! Of course it was a stupid thing to do, but that's what being young is all about.