Below is code to make this happen. I left out the || reading logic for making a separate argument list.

(defun underscore-num-sym-p (sym)
(and (symbolp sym)
(let ((str (symbol-name sym)))
(string= “_” str)
(parse-integer str :start 1 :junk-allowed t))))

;; Used to make two closures one that
(defun make-symbol-finder ()
(let ((max-sym 0))
(values
(lambda (sym)
(let ((result (underscore-num-sym-p sym)))
(and result (> result max-sym) (setf max-sym result))
result))
(lambda () max-sym))))

;; (flatten ‘((1 2 3 nil 4 5 (nil 2 nil))))
;; Graham: (1 2 3 4 5 2)
;; this: (1 2 3 NIL 4 5 NIL 2 NIL)
(defun flatten (x)
(labels ((rec (x acc)
(cond ((atom x) (cons x acc))
(t (rec (car x) (if (cdr x) (rec (cdr x) acc) acc))))))
(rec x nil)))

(defun bracket-reader (stream char)
(declare (ignore char))
(multiple-value-bind (lsymbol-p num-lsyms) (make-symbol-finder)
(let* ((body (read-delimited-list #\] stream t)))
(remove-if-not lsymbol-p (flatten body))
`(lambda ,(loop for i from 1 to (funcall num-lsyms) collect
(intern (concatenate ’string “_” (write-to-string i))))
,body))))

(set-macro-character #\[ #’bracket-reader)
(set-macro-character #\] (get-macro-character #\#))