You realize that the system to deal with this already exists? It's called the legal system, and it hooks into various specialized professions, including medical ones. As in, the partner brings the lawyer some papers from doctors that say you're out of commission, and the lawyer executes whatever procedure you prescribed for such a case. The same way as when you're dead, just without the death part (hopefully).
(Cue HNers saying that the papers can be forged, etc.—if that's the worry, one has more immediate problems in their relationship than the arrangement of a ‘dead man switch’.)
Sometimes I have to wonder if a bunch of hackers from ~2000s decided to live entirely in computer systems, and forgot that the ‘hacker ethos’ pertains also to researching and using existing systems.