I suspect any other "hook" would work just as well, a comment with a nonce--and could serve as block boundaries to make changes more likely to be complete?

Graphologue used a version of this too: https://hci.ucsd.edu/papers/graphologue.pdf

This is actually a very powerful pattern that everybody building with LLMs should pay attention to, especially when combined with structured outputs (AKA JSON mode).

If you want an LLM to refer to a specific piece of text, give each one an ID and then work with those IDs.