###やりたいこと###
CookieのNameと値を変えたり，httponlyをtにしたりしたい。

###app.lisp###
とりあえずapp.lispのbuilder内部の<clack-middleware-session>が，外部からいじれる部分だろうと思って<clack-middleware-session>をsuperclassとしたclassを定義してみました。

(defclass <myapp-clack-session-state-cookie> (<clack-session-state-cookie>)
  ((session-key :initform :myapp.session
		:accessor session-key)
   (secure :initform nil
	   :accessor secure)
   (httponly :initform  t
	     :accessor httponly)))

(defclass <myapp-clack-middleware-session> (<clack-middleware-session>)
  ((state :initform (make-instance '<myapp-clack-session-state-cookie>)
	  :accessor state)))

(builder
 (<clack-middleware-static>
  :path (lambda (path)
          (if (ppcre:scan "^(?:/images/|/css/|/js/|/robot\\.txt$|/favicon.ico$)" path)
              path
              nil))
  :root *static-directory*)
 (if (productionp)
     nil
     (make-instance '<clack-middleware-accesslog>))
 (if (getf (config) :error-log)
     (make-instance '<clack-middleware-backtrace>
                    :output (getf (config) :error-log))
     nil)
;; <clack-middleware-session>
 <myapp-clack-middleware-session>  ;;ここだけ書き換え
 (if (productionp)
     nil
     (lambda (app)
       (lambda (env)
         (let ((datafly:*trace-sql* t))
           (call app env)))))
 *web*)

なんてしてみたものの変化なし。

試しに直にclack.session.stateのsession-keyの値を書き換えたらcookieの値も変わりました。

(defclass <clack-session-state> ()
     ((session-key :type keyword
                   :initarg :session-key
                   :initform :myapp.session ;; :clack.session
                   :accessor session-key)
      (sid-generator :type function
                     :initarg :sid-generator
                     :initform
                     #'(lambda (&rest args)
                         @ignore args
                         (clack.util:generate-random-id))
                     :accessor sid-generator)
      (sid-validator :type function
                     :initarg :sid-validator
                     :initform
                     #'(lambda (sid)
                         (not (null (ppcre:scan "\\A[0-9a-f]{40}\\Z" sid))))
                     :accessor sid-validator)))

=> MYAPP.SESSION e3d5067aceb466a9b01a04deb2170137c8bec3b0

CLOS自体も大分忘れてるので混乱中…
とりあえずもうちょっとソース読み込みます…

###追記(2015-01-05-1310)###
builderを以下の様に書き換えたらできました。

(builder
 (<clack-middleware-static>
  :path (lambda (path)
          (if (ppcre:scan "^(?:/images/|/css/|/js/|/robot\\.txt$|/favicon.ico$)" path)
              path
              nil))
  :root *static-directory*)
 (if (productionp)
     nil
     (make-instance '<clack-middleware-accesslog>))
 (if (getf (config) :error-log)
     (make-instance '<clack-middleware-backtrace>
                    :output (getf (config) :error-log))
     nil)
 ;; <clack-middleware-session>
 (<myapp-clack-middleware-session>
  :state (make-instance '<myapp-clack-session-state-cookie>))
 ;; <myapp-clack-middleware-session>
 (if (productionp)
     nil
     (lambda (app)
       (lambda (env)
         (let ((datafly:*trace-sql* t))
           (call app env)))))
 *web*)

<myapp-clack-middleware-session>の:initformが効いてない？？

とりあえず出来たけど疑問が残ってます。