I don't know how much reaction time you had, but remember you have an emergency brake and the main brakes can still be pumped mechanically to stop. You don't have to slam it and send your car into a skid, but you should be able to depress it enough to start the mechanical braking to get the car to stop (keep the foot on the main brake of course).
Now, that aside, I agree, very disturbing issue. Was the vehicle nearly fully charged when you went down this very steep hill? It's been known that when the vehicle has a full battery, you can't regen very much into the battery, even in B mode. This is to protect the battery for over-charging. Now, even if the regen kicks out, your mechanical brakes should still work as they are like the power brakes on any car. The only thing that would kill those is if the brake pump were to turn off. It does sound like car hit some type of shutdown mode if both Regen and Power Brakes failed.
How fast were you going down this steep hill? Was the Regen B mode helping you coast at a consistent speed or were you speeding up the entire way down the hill (before things shutdown). I actually have a long "straight" section of about a 1 mile road that goes down with this grade level near where I live. If you can give a good description of everything you remember, I might be able to perform a similar test but with no killer road curves at the bottom.
Also, where in the country do you live?