Freespeed2,
I've never heard of 100% being an issue for well executed emergency descent. If sustained unpressurezed flight at those levels, then yes 100% may not provide sufficient O2 for life.
You would hope that once you'd recognised the ECAM, donned mask, established comm, announced emergency descent, flicked the seatbelt sign on, and initiated the descent, the aircraft would be approaching the mid 20's within 2 1/2 mins and the 100% would have been enough to get you there without hypoxia setting in.
The primary function of the 'Emergency' setting as far as I know is for keeping the mask free of smoke. I'm certainly open to others opinions on this? Do I need to use 'Emergency' O2 at FL380???