I came accross this Prince manual page for the prince two-pass solution with the application of generating an index with page reference numbers:
https://www.princexml.com/doc/two-pass/#two-pass
The prince two-pass solution might help with the index featute mentioned on the Pressbooks Roadmap.
The Prince example assumes that the index terms (one level) are marked with span class=“ix” tags and that the provided JavaScript is applied during two-pass processing with Prince.