Adding support for the above is pretty easy if we drop support for link bidirectionality for callout markers (which doesn't work anyway if there's not a 1-to-1 correspondence between markers in listings and markers in callouts below) and just have the callout markers in the listing link to the callout entries below the listing.
Right now, HTMLBook backends don't support markup like the following, which contains a repeated callout ref:
Adding support for the above is pretty easy if we drop support for link bidirectionality for callout markers (which doesn't work anyway if there's not a 1-to-1 correspondence between markers in listings and markers in callouts below) and just have the callout markers in the listing link to the callout entries below the listing.