Commenter Alphonsus asks, "Given that capital punishment is legal, would it be possible for the government to force a given doctor to perform an execution if no other doctor was willing?"
This question is confused on a number of points. Superficially, the law is at best an imperfect mirror of ethics (and at worst and usually simply a codification of class oppression). It's physically possible for the government to force most anyone to do most anything; the question is: is it ethical to do so?
More deeply, though, in discussing abortion specifically, we're asking about the best interests of the patient. There are two general ethical questions relating to abortion. First: is the fetus a patient whose best interests a physician is ethically obligated to act? It's notable that Bob Hunt does not address this particular question in his opinion. He asks instead: if a physician (or other medical professional) believes that some treatment is indeed in the best interest of the patient, but the treatment violates the physician's own personal moral beliefs, is the physician is entitled to deny that treatment? He answers in the affirmative.
(Another question is: to what degree does a woman forfeit any of her ethical rights by getting pregnant? It's entirely contemptible to answer this question in any way except, "Not at all.")
A condemned prisoner is in an objective sense a sapient human being. Regardless of legal, social and ethical considerations, it's objectively true that a prisoner has interests, and it is never in his interest to be executed (assuming he does not wish to commit suicide). Therefore a physician cannot act in the prisoner's best interests by participating in an execution, and therefore the physician should not be compelled to do so. Indeed, medical ethics — if constructed in terms of the patient's best interests — would forbid participation in an execution. In Mr. Hunt's own terminology, a physician would indeed become a "tool of the state", acting in the state's interests instead of the patient's.
Thus Alphonsus asks a substantively different ethical question: answering one (on any basis) does not in any way suggest any particular answer to the other.