We think it would be a matter of simply asking the patient. Presumably the patient is in a conversation with the doctor who is refusing, a doctor who, under the norms of informed consent, would have had to provide information about the treatment. We would not excuse doctors from that obligation.
It would simply be a matter of saying to the patient, “May I indicate my refusal to this body?” If they say yes, then the doctor can do that. If they say no, then the doctor doesn't have the permission to make that indication.