I get the sense, though, that if someone is dead, it's no longer a good assumption that everywhere important in their brain is close enough to navigable blood vessels
Exactly. Keep in mind also that blood vessels are not steel pipes, it is very difficult to keep capillaries open, and the circulatory system also relies heavily on the pumping action by the blood vessels themselves.
Between this and the protein denaturation by the "cryoprotectants" (and/or by the concentrated brine that you get when water freezes out first), to the best of current knowledge the frozen bodies are irreversibly dead irrespective of any future scanning or simulation technologies (barring fundamental discoveries in physics, at which point you could as well hope for your brain to be scanned via some sort of look-into-the-past wormholes).
to the best of current knowledge the frozen bodies are irreversibly dead irrespective of any future scanning or simulation technologies
I do not believe this individual is in possession of the best of current knowledge or at all capable of judging the capabilities of all future scanning or simulation technologies.
In the past, the Cryonics Institute has had a policy that said that they would not accept anyone who is not a member. This has changed. The policy now is that someone who has full legal authority over your body can sign you up after you die. It costs $36,250 to be frozen if you are not signed up, which is more expensive. They also will not do anything until you have been on dry ice for 2 weeks after they have been contacted, so not being a member is more risky.
This is very important news for anyone who is currently cryocrastinating. It means that you can drastically increase your chances of survival without filling out any forms. All you have to do is tell a loved one you want to be frozen upon death, and that you would like them to take responsibility for making sure this happens. This takes literally 30 seconds. Do it now!
This news might also be a reason to not sign up right away, if you think something better (like radical life extension or uploading) will come along in your lifetime. We should discuss this in the comments.
Edit: The general consensus of this discussion is that this is a really bad reason not to sign up for cryonics.