I poked a round a bit in the source (for instance here), but it wasn't clear to me where I might hook in to add this functionality. Pointers to the correct location would be appreciated! I might be able to put together a PR.

My use case is Hakyll, where I want to use Pandoc to generate a bibliography, but I want a hyperlinked title instead of raw URLs. I could hack something together to modify the HTML manually, but since there's an official CSL recommendation, it might be a nice feature to have in Pandoc itself.

For reference, here is a snippet of the mentioned spec:

If the bibliography entry for an item does not render any of the above identifiers (doi,pmcid,pmid,url), then set the anchor of the link as the item title. If title is not rendered, then set the anchor of the link as the full bibliography entry for the item. Set the target of the link as one of the following, in order of priority:

doi: prepend with "https://doi.org/"
pmcid: prepend with "https://www.ncbi.nlm.nih.gov/pmc/articles/"
pmid: prepend with "https://www.ncbi.nlm.nih.gov/pubmed/"
url: output as is

If the item data does not include any of the above identifiers, do not include a link.