Why would you not breath off the BC inflator? This is a viable Plan C even for standard inflators
I don't think I would attempt to do this at depth (although I suppose it would depend upon the situation) as I have had some blow-back of water from the BC and would run the risk of aspiration. If an individual was comfortable with the procedure, it would be a viable option, but to be comfortable you have to practice and because of the molds that tend to grow in the BC bladder, practicing may not be the healthiest thing to be doing.
I am a strong supporter of CESA and practice it on most dives. If I have one breath of air, I'm good to go to the surface or the decompression stop where there's gas waiting. It is however a last resort. I would much rather CESA than be holding my breath and fight over one functioning regulator.