>If I were to make this usable for me, it's probably by automatically keeping record of what buffer I was on when I opened a new one, then making a buffer that presents this information in a modifiable tree.

This is already available in the form you described as Global History Tree. See: https://nyxt-browser.com/article/global-history-tree.org

But this is only a presentation and no management (currently) exists. A 1-1 window/buffer map is (also) an open issue.