I found some existing mpv scripts:

https://github.com/oltodosel/mpv-scripts/blob/master/show_ch... display chapter names as OSD

https://gitlab.com/lvml/mpv-plugin-excerpt press 'i' and 'o' for in/out points, then 'x' to make a new (auto-named) file.

https://github.com/shinchiro/mpv-createchapter press 'shift-c' to mark chapters; export as xml file.

https://github.com/mar04/chapters_for_mpv mark chapter times, input titles, save as txt file

By having an LLM name the chapters, I meant having whisper do speechrec on the chapter and then asking an LM to summarize the content into a name up to k chars.