For those who might be interested; this is what I came up with:

(defun hash-table-to-alist (hash-table)
  (loop for key being the hash-keys of hash-table
      using (hash-value value)
      collect (cons key value)))

(defun hash-print (s h)
  (write-string "#h" s)
  (write (cons (hash-table-test h)
               (hash-table-to-alist h))
         :stream s
         :pretty t))

(defun hash-read (stream subchar arg)
  (declare (ignore subchar arg))
  (destructuring-bind (test &rest entries) (read stream t nil t)
    (let ((tab (make-hash-table :test test)))
      (loop for (key . value) in entries
          do (setf (gethash key tab) value))
      tab)))

(let ((pprint-dispatch
       (let ((tab (copy-pprint-dispatch nil)))
         (set-pprint-dispatch 'hash-table #'hash-print 0 tab)
         tab))
      (write-dispatch
       (let ((tab (copy-pprint-dispatch nil)))
         (set-pprint-dispatch
          t #'(lambda (s obj) (write obj :stream s :pretty nil))
          0 tab)
         (set-pprint-dispatch 'hash-table #'hash-print 1 tab)
         tab)))
  (defun mi-write (obj &rest args)
    (let ((*print-pprint-dispatch*
           (if (getf args :pretty *print-pretty*) pprint-dispatch
             (progn (setf (getf args :pretty) t) write-dispatch))))
      (apply #'write obj args))))