Wow! Run out of air at 100’ and have to breathe off your BC air to make it to the surface.
I would think the prudent thing to do would be to train divers extensively (drilled into their heads) to check their air supply and keep their stuff serviced and in good shape. Buddy or no buddy, no excuses!
I wonder if pilots are trained to look at their fuel gauge and keep their planes maintained, or do they just carry a parachute and have to jump every once in a while?
Well of course Eric...We all know how divers should be trained and that they shouldn't do stupid things like ignore their gas supply. What I'm talking about here is some unforseeable catastrophic failure that would put someone in that position. Maybe an actual problem with the reg and it is that one in a million times it doesn't free flow. Maybe some sort of entanglement--I can't think of other possibilities. But it's like a CESA from 30 feet-- you SHOULD never have to do that, but it's nice to know.
If someone is stupid enough (or has narcosis) at 100' to not constantly be checking remaining gas they are probably not swift enough to consider breathing from the BC to ascend anyway. They may be diving solo. Problem solved. Darwin.
Now, re never breathing from the BC...
This is something, as I said, I would never practice. CESA OTOH, is something I may do from 20-30' on every other dive, simply because I usually ascend at some point to check my exact location, so why the heck not. AND obviously, I know how to do it safely.
But, I have to figure (and may well be wrong) that if you're down 100+ feet and OOA yes, you can do a CESA. And the air in your lungs will increase in volume. But, I think it may be that you can't inhale any fresh O2 for your tissues. That is bad, depending on how slowly you ascend. The urge to breathe caused by C02 may also cause you to think "hey, what about the BC air?"
All irrelevant stuff if you are prudent and check your gas supply--and your gauge is not stuck on a number, like mine was once stuck on 1000 PSI. It was good I was doing my usual shallow diving that day.