Very difficult situation which I may have to face myself at some point with my very elderly JW father. I would be inclined to speak frankly with your JW relative if you think this could come up. It is their responsibility to complete the advance directive, and speak with their doctor about their wishes. If they fail to do these things, I don't think its your responsibility to do it for them if you are conscientiously opposed to whatever the current WT policy might be. If they are unconscious, and have not cared for these things in advance, I would assume they privately have serious reservations that they have not expressed.
You are left with two choices in my view:
1. Say nothing other than "please provide the highest level of care for my.....(relative).
2. My (relative) is a JW. However, they may have had serious reservations about supporting the policy since they did not complete the Watchtower's advance directive, and make necessary arrangements through their primary care doctor and/or specialist.