This came up for my mother a few years ago. After years of no contact, I was asked by social services to assume a medical power of attorney (the UK term) over my mother.
Ultimately, I refused to accept, and suggested that the Hospital Liaison Committee select an elder (by coincidence, the chair of my DC) to assume that responsibility, as I didn't wish to risk being in the position of having either to uphold the blood doctrine, or override what I knew her express wishes to be. Were she younger, or were her long term prospects better, perhaps I would have been willing to intervene, and would have accepted, but in her current condition, I felt it would do more harm to her if she lived having been transfused, than had she died as a result of refusal.