手抜き。あとvだとMIMEパートの表示とかぶるのでbで

(defun wl-message-bury-message ()
(interactive)
(wl-message-exit)
(wl-summary-toggle-disp-msg))

(add-hook 'mime-view-mode-hook
(lambda () (local-set-key "b" 'wl-message-bury-message)))