My institution has a few specific citation style requirements that stray from the standard Chicago Style. The changes are mostly around specific works, so it's difficult-to-impossible to meet the requirements by making a new CSL.
For the most part I'm able to handle it with Lua filters. I intercept the citation reference both before and after --citeproc runs and make whatever changes I need to; this might entail changing the style to SuppressAuthor, making some capitalization/punctuation changes, etc. In general this works well, but where it gets difficult is if a single Cite element contains multiple citations. In that case, looking at the generated footnote, there's no hard indicator of which portion came from which citation. I've got something working based on scanning for semicolons, but I worry that's a little bit fragile and will probably break on certain sets of reference data.
Is there any good way of mapping portions of the footnote to citation keys? I was thinking to wrap them in a span with some custom metadata, but I can only see the citations either before --citeproc (when the footnote doesn't exist yet) or after (when the multiple citations have already combined). What would be truly ideal would be the ability to filter the citation process itself, but I'm assuming since that's from a different library that it is not exposed to Lua?
Like I said, I have something mostly working now, but any thoughts/advice would be appreciated.