convert-to-stings

#+source: hetero-table
#+begin_src emacs-lisp
  '((1 2 3 4)
    ("a" "b" "c" "d"))
#+end_src

#+source: all-to-string
#+begin_src emacs-lisp :var tbl='()
  (defun all-to-string (tbl)
    (if (listp tbl)
        (mapcar #'all-to-string tbl)
      (if (stringp tbl)
          tbl
        (format "%s" tbl))))
  (all-to-string tbl)
#+end_src

#+begin_src emacs-lisp :var tbl=hetero-table
  (mapcar (lambda (row) (mapcar (lambda (cell) (stringp cell)) row)) tbl)
#+end_src

#+results:
| nil | nil | nil | nil |
| t   | t   | t   | t   |

#+begin_src emacs-lisp :var tbl=all-to-string(hetero-table)
  (mapcar (lambda (row) (mapcar (lambda (cell) (stringp cell)) row)) tbl)
#+end_src

#+results:
| t | t | t | t |
| t | t | t | t |