(defmacro define-binary-type (name (&rest args) &body spec)
  (alexandria:with-gensyms (type)
    `(progn
       ,(destructuring-bind ((in) &body body) (rest (assoc :reader spec))
          `(defmethod read-value ((,type (eql ',name)) ,in &key ,@args)
             ,@body))
       ,(destructuring-bind ((out value) &body body) (rest (assoc :writer spec))
          `(defmethod write-value ((,type (eql ',name)) ,out ,value &key ,@args)
             ,@body)))))

(trivial-indent:define-indentation define-binary-type (4 4 &rest (&whole 2 4 4 &body)))

;; produces the following indentation:

(define-binary-type iso-8859-1-string (length)
  (:reader (in)
    (let ((string (make-string length)))
      (dotimes (i length)
        (setf (char string i) (code-char (read-byte in))))
      string))
  (:writer (out string)
    (dotimes (i length)
      (write-byte (char-code (char string i)) out))))