I have a dirty book-to-slideshow transformation here:
https://github.com/TeamBasedInquiryLearning/linear-algebra/blob/main/slides/extract.xsl (which produces PreTeXt slideshow XML, which can then be built to Reveal.js).
It's... not great! That book was originally a slidedeck, so everything was designed with slides in mind. Even still, some slides look better than others.
I think what you describe Runestone has is closer to what we need for "book to slideshow", and have imagined something similar to what you describe. With the React.js overhaul, maybe this can be recovered.