On a long downhill it's not going to be fun. A braking system holds only one maybe two pumps on the brakes in reserve when the engine quits. After those two pumps you're down to non assisted braking. As stated above you might need both feet on the pedal at that point, but the brakes will still work.
I have no idea if it would work. But if the engine quits you might try shifting the automatic transmission down to a lower gear. Worst case nothing happens as the transmission is freewheeling. Best case is you get some engine compression braking to help with keeping everything under control.