>> (On the other hand, if someone were to work toward a `cl-format',
>> even one that was partial and could be extended later, I'd likely
>> welcome that.)
>
> There is one available on MELPA already, under the name ‘cl-format’.
> Some basic testing seems to suggest that it works reasonably well:
> (cl-format nil "~{~s~^, ~}" '(1 2 3 4)) --> "1, 2, 3, 4"

Great.  Perhaps the author will agree to including it in GNU Emacs
(signing papers etc.).