Could this deficiency be caught by some kind of blood test, though? Do regular checkups screen for this? Is there some reason you'd be "super low on it in brain tissues" and not in other parts of the body?
See e.g. here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6392124/
That's why I think people should just try megadosing B1 for a while as it is not toxic and to get a datapoint if their troubles can be related to a deficiency somewhere. Many people get instant relief of their problems just after a single dose but the effect might get weaker as the supplementation continues. However, it's a good hint for their physician where to look at.