Thanks for digging into this, Ihor.  It exports now, but I'm afraid it's 
doing too much capitaliztion.

A name like "van Dongen" should become "Van Dongen" when cited with a caps 
variant, for example [cite/a/c:@friends].  This is turning it into "VAN DONGEN", 
upcasing the whole string when it should just be doing the first letter.

I looked into how the CSL versions handles it.  In citeproc-el's 
citeproc-site.el there's this:  "CAPITALIZE-FIRST is non-nil if the first word 
of the rendered citation should be capitalized."  That goes with this in 
oc-csl.el in the definition of org-cite-csl--create-structure-params for doing a 
/a/c author-caps citation:

    ((or "caps" "c") '(:mode author-only :capitalize-first t))

So for the basic exporter to do what the CSL one does, it should just capitalize 
the first letter.  I hope you don't mind opening up that file one last time for 
this fix ...

Thanks,

Bill

--
William Denton
https://www.miskatonic.org/
Librarian, artist and licensed private investigator.
Toronto, Canada
CO₂: 420.04 ppm (Mauna Loa Observatory, 2023-12-01)