Redirect a stream to Emacs buffer.

stream-to-emacs-buffer.lisp

;;; Remember to set `slime-enable-evaluate-in-emacs' to t in Emacs.
;;; Also remember that it basically allows your Lisp to execute any code in your Emacs.
(defmacro with-output-to-emacs-buffer ((stream buffer-name) &body body)
  `(let ((results (with-output-to-string (,stream)
                    ,@body)))
     (swank:eval-in-emacs `(with-current-buffer (get-buffer-create ,,buffer-name)
                             (erase-buffer)
                             (insert ,results)))))

;;; Use example:
(with-output-to-emacs-buffer (*standard-output* "Your Buffer Name") (some-call-that-prints-to-stdout))