I realize, but it's still a pain to do, and how to actually configure TailScale like this is completely undocumented, I couldn't find any mention of this featur

I realize, but it's still a pain to do, and how to actually configure TailScale like this is completely undocumented, I couldn't find any mention of this feature outside of a hacker news comment. On the OSX application there's a hidden menu (hold option) which allows you to choose a different master server, but not enter your own, which is so close to being usable but just out of reach.