"This is obviously why the official line from Mill Hill is that blood transfusions are a decision for each individual. If a JW was to get DFd for giving consent for their child to have a transfusion it could make an intresting test case."
Unfortunately this will never happen. No JW is DF'd any more for accepting a transfusion. Instead it is decided that the person has disassociated themselves by their actions. Of course, the announcement is identical and they will be shunned in just the same way, but the ORG can continue to claim they did not DF the ex-member.
The way it works these days in the Org., everyone makes their own personal choices and is responsible for the results. Of course, if you make a choice the Org. does not approve of you will be DF'd or disassociated so there is not really a choice at all. It's the same way they teach their God works. Everyone has the freedom of choice to worship Jehovah or not - but of course they will die if they choose not to.