It is not impossible actually.
The case that I presented to you is loosely based on a real incident that happpened in my extended family.
I remember that my family was told that laws preventing adults from refusing any medical treatment are unconstitutional.
In other words, the right for an adult to refuse medical treatment, through his guardian, has been established in our courts and could not be overturned by legislative action.
My memory is not entirely clear on this issue, so that's why I posited the question.
Of course, it can be argued that IV hydration therapy and/or a feeding tube are not medical treatments.
At least some precedent has been set... a law tailored to a particular patient will be passed if enough raucus is raised.