Why would your organs become less 'fresh' if you are stable but appear to be in a vegetative state? I understand being worried about people taking advantage of disabled (perhaps temporarily) folks - but I don't think there's a ticking clock issue here?
If you're already (physically) dead by the time you arrive at the hospital, the chances of successfully harvesting a working organ are low.
Thus, organ harvesters prefer patients who are physically alive but brain dead. Organ donor status basically gives them permission to harvest your still-beating heart.
The problem isnt if you are actually brain dead. The problem is if you just appear to be brain dead. Organ harvesting is done without the aid of anaesthetic.