> Even good human writers would probably struggle with this constraint.

But that would be a fun writing exercise, I think. Thoroughly in the oulipo wheelhouse.

Maybe generate a Markov chain table over all of Project Gutenberg and then say every 10th word is whatever the Markov Chain thinks it should be at that point?

Or every Nth word has a P% possibility to be constrained by the chain? Optionally with the possibility building for each skipped word to guarantee it happens at some point. Bonus with this approach is that the human can't game the words leading up to the constraint because you don't know when it will happen.