I have the function now working to my satisfaction, as below, but am open to suggestions if I'm not doing things optimally. In the solution below, I use function read-file-name to prompt the user, and use condition-case to trap a user response of C-g, the quit signal, so that the default buffer file name is set for the buffer.

(defun my-write-test ()
  (interactive)
  (let*
     ((name "foo.txt")
      (dir "/home/me/")
      (buf (create-file-buffer name)))
   (pop-to-buffer buf)
   (insert "random text\n")
   (condition-case nil
     (progn ()
       (set-visited-file-name
         (expand-file-name
           (read-file-name "Save to: " dir nil nil name)))
       (save-buffer))
     (quit (set-visited-file-name name))))))

I have the function now working to my satisfaction, as below, but am open to suggestions if I'm not doing things optimally. In the solution below, I use function read-file-name to prompt the user, and use condition-case to trap a user response of C-g, the quit signal, so that the default buffer file name is set for the buffer.

(defun my-write-test ()
  (interactive)
  (let*
     ((name "foo.txt")
      (dir "/home/me/")
      (buf (create-file-buffer name)))
   (pop-to-buffer buf)
   (insert "random text\n")
   (condition-case nil
     (progn ()
       (set-visited-file-name
         (expand-file-name
           (read-file-name "Save to: " dir nil nil name)))
       (save-buffer))
     (quit (set-visited-file-name name)))))