Shirley the most dangerous part of this would have been the first landing at the top of the mountain?
The first time he landed at the top it would have been onto an unknown surface with all of the risks that that would carry. Once on the top a reccy, on foot, could be used to check the surface for hidden boulders/holes/sharp obstructions etc and a best way for take off judged.
The engine off departure could be worked up to by taking off the first time with normal full power setting and then repeating at 75%, 50%, 25% and finally, engine off. Once at this point there is little risk of EFATO cocking things up so it is probably safer than level ground with a hedge at the end.
In my experience a downhill take off is pretty much a non event but the uphill landing still causes me to "grip the button in the middle of the seat cushion" if there is no go around option at the landing site.
Rans6...