If a human is really nonsentient–not conscious or aware of anything at all and will not regain consciousness or awareness of anything–then, by definition, the human cannot have an interest in not suffering (or in anything else). In such a situation, a compelling argument could be made that it is morally acceptable to use the organs of such a human to save others–and it is common practice to do so if the human has previously agreed to donate her organs or if the family consents.
We should, of course, be concerned about whether an ostensibly brain-dead human really does lack all cognitive activity. We ought also to be sensitive to the concerns of those related to the comatose human; they may oppose the instrumental use of the human for various reasons, such as religious opposition to organ transplantation. But humans who are really irreversibly brain dead are really no different from plants; they are alive but they are not conscious and have no interests to protect. According such humans a basic right not to be treated as the resources of others makes no sense.