Indeed, but if control and resources (e.g. immune resources) are allocated via some kind of map in the brain then mightn't that make proximity relevant? (notwithstanding the bone barrier!) I admit it's a stretch...
There is a pretty large concentration of veins/arteries - 1 that come up around the temple, so that could be related to the frequency of graying starting there.
1- https://www.gettyimages.com/detail/news-photo/superficial-ar...