I find it hard to believe that the WTS will put anything in writing on these healthcare choice documents that compels a jw to make a certain choice. Put it behind the scenes more likely. jws don't know what the meaning is a the medical choices, many just blindly do what they are told verbally by elders, COs, etc.
I'm sure the letter doesn't say "make sure every publisher signs it or they're DFed" - it'll say that they've made "provisions" for people to keep their card on file with the elders, and are encouraged to do so. We all know what it means, though. The elders will be "encouraged" to verify that they have something on file for every publisher to make sure that the publisher was aware of this provision (wouldn't want them to miss out just because they didn't know about this loving arrangement!) but what it really means is that they're supposed to hound everyone until they've all signed away their lives and have it on file. When they put coercion in writing, they do it subtly and in a way that they can claim it means something else.