Coded some of the semi-tricky bit involving geography repositioning itself for dramatic purposes in one of the endgame worlds (let's call it "Reb" for now). I managed to uncover two new Inform 7 bugs in the process, both for semi-obscure situations. Fortunately nothing show-stopping. Reb's spine is almost done, though fairly threadbare and pitiful; I keep stopping myself from diving into details.
The only thing left to code is some of the machinery to handle leaving and repeat visits and what happens if the player leaves in the middle of X, and so on. The design decision to keep from restricting the player for arbitrary reasons is sound, but it makes for some extra work to account for situations that a lot of designers would just avoid with a "You're not going to do that" message.
It will be worth it. Wherever there are rails, I'm trying my best to hide them.