You can achieve this with the existing spec, just use the "toc:false" attribute to hide these chapters from each app’s table of contents. https://github.com/Podcastindex-org/podcast-namespace/blob/main/docs/examples/chapters/jsonChapters.md
btw, the use case you’re describing is exactly why Overcast doesn’t support JSON chapters. He sees this as basically the same as NPR’s RAD.