Hi,

So I found this other thread

  https://lists.gnu.org/archive/html/emacs-devel/2017-12/msg00667.html

where plist support for json.c is briefly requested and discussed, but
there didn't seem to be an overwhelming argument against it, so I had
another look at the file and it didn't seem very hard or problematic to
implement.

Anyway, I had a go.  It's small, so have a look.  Patch at the end of
this file, or find it in the scratch/support-plists-in-jsonc branch.

I used a global to control json-serialize's interpretation of lists, but
it could be an argument as well.  Added some tests and some doc, too.

João

PS: Take this opportunity to thank Eli and everyone else very much for
the Emacs 26.1 release!