How to match everything except a particular pattern after/before a specific string constant

You can use an anchored font-lock rule. It's like a search within a search. The code below searches for AST(. Once it's found, it find the end of the argument list and performs a search for all words within it.

(defun foo ()
  (interactive)
  (setq font-lock-multiline t)
  (font-lock-add-keywords
   nil
   '(("\\_<ATS("
      ("\\_<\\sw+\\_>"
       ;; Pre-match form -- limit the sub-search to the end of the argument list.
       (save-excursion
         (goto-char (match-end 0))
         (backward-char)
         (ignore-errors
           (forward-sexp))
         (point))
       ;; Post-match form
       (goto-char (match-end 0))
       (0 font-lock-builtin-face))))))

This will match all words inside the parentheses, even if they are spread out across multiple lines.

The pre-match form has two purposes: It can position the point to a suitable locations and it controls the extend of the sub-search (where nil means the end of the line). The post-match form can, for example, be used to return the point to a good position for other keyword rules.

This can, of course, be extended to only highlight a specific set of words, but I leave that as an exercise.