The lack of demand tells me that this isn't something I should build, at least as described. This is expecially true given you can get the same behavior by creating a composite column, hiding it in the book view, and using the new "Show with comments in book details" option.
However, I am considering an alternate approach: providing a way to register a "generator function" for a composite column. If this works out then when the value of a composite column is required, calibre calls the generator function instead of calling the template processor. The generator must return a string matching the composite column type definition: date, number, or text. If text then it can be HTML, markdown, or plain text.
This scheme allows a plugin to do more-or-less arbitrary computations to produce the column value. For example, it could display:
- an HTML table of links for the book.
- bibliographic information about the authors
- timelines for series
- information about book editions
- formatted bookmarks
- ...
Because these are normal composites they would work in search and sort, as well as be displayable in book details. They could be displayed in the book list if the callback is fast enough.