AFAIK mine does it automatically (i.e., when driving down a slope, it engine brakes to avoid acceleration). Then again we don't have that many slopes so maybe I misinterpreted what was happening the few times I drove down a parking garage :)
I understand why: it's most efficient to drive in the highest gear even at speeds as low as 45 mph in a manual. I would imagine it's similar for automatics too.
Ford trucks with a Tow/Haul button enabled will do exactly what you describe though, especially if you tap the brake for an instant while going downhill.