A person in solitary still has experiences. They just don't interact with the outside world. People in a coma are, as far as we can tell, not conscious. There are plenty of animals that people are okay with killing and eating that are more likely to be sentient than someone in a coma.

Given that this is suggested to be a voluntary system, it doesn't really matter what Walter Glannon thinks -- it matters what you think.

Personally, I would be more interested in signing up for this if I was assured that the permanent damage was to the grey matter, and would be happy if this included both comas and permanent vegetative states. But YMMV.

It is worth noting here that being in solitary confinement does not necessarily prevent you from writing, receiving visitors, or making telephone calls (it depends on your local jurisdiction). Also, very few people are sentenced to be in solitary confinement until they die. In those places where this sort of sentence is permitted, it is unlikely that prisoners would be allowed any choice in their fate, but it is not obviously bad for a justly imprisoned person to choose suicide (with or without organ donation) in lieu of a life sentence.

EDIT: on re-reading, I see that this was not stated to always be a voluntary procedure; the author goes back and forth between voluntary and involuntary procedures. In involuntary cases, I agree that the simple criteria of "brain functions at a level too low to sustain consciousness but enough to sustain breathing and other critical functions without mechanical support" is too lax. I would still agree with the author in general that DDR is too strong.