As has been said elsewhere ‘clinically dead’ doesn’t mean ‘brain dead’. OK in effect it does, but in actuality there can still be brain activity in ‘clinically dead’ patients, but no where near enough to keep the patient running if they weren’t on machines. So there is a good chance that someone classified as ‘clinically dead’ might still have a few neurons firing in their head and who knows what that will be perceived as?
If you have a bonfire and in the morning you survey the ashes you would say it was out, or dead, no more fire. But if you give the ashes a poke and look in side there can frequently be a few glowing embers yet remaining.
Unless you scan the brain of a dead/clinically dead patient with CT and MRI scanning you can’t say for sure absolutely ALL activity has ceased. So if you can’t say for sure that there is absolutely no brain activity you can’t make a claim for this kind of experience.
Even if you do manage to accurately ascertain there is absolutely no brain activity at all for any period of time, you can’t prove that the experiences remembered by the patient were actually experienced during the period of recorded total inactivity.
Human perception of time flow and events can become distorted and confused, especially under extreme stress and I would be hard pressed to identify a bigger stress than death.
The only way you cold prove that humans can perceive events post actual death (zero brain activity, not just ‘clinical death’) is to scan a patient to see if there is even the slightest activity, if not, if there is absolutely no brain activity, at any level then try reading them a unique piece of text. If they re-awaken and can recall the text there might be a case for OoBE and NDE experiences. Otherwise you can’t rule out the chance that there is still an extremely low level of activity that is giving rise to these perceived experiences.
My money would be on there still being some level of brain activity however slight in all cases where patients came back and reported an experience. And if there is any level of activity then they are not totally dead and thus you can’t claim a consciousness separate from the brain/mind.