プロンプトを自動的に受け入れる変数がないようです。
ffap-read-file-or-url
プロンプトを実行している部分を削除することで、関数を再定義できます。こんな感じで残っています
(defun ffap-read-file-or-url (prompt guess)
"Read file or URL from minibuffer, with PROMPT and initial GUESS."
(or guess (setq guess default-directory))
(let (dir)
;; Tricky: guess may have or be a local directory, like "w3/w3.elc"
;; or "w3/" or "../el/ffap.el" or "../../../"
(or (ffap-url-p guess)
(progn
(or (ffap-file-remote-p guess)
(setq guess
(abbreviate-file-name (expand-file-name guess))
))
(setq dir (file-name-directory guess))))
;; Do file substitution like (interactive "F"), suggested by MCOOK.
(or (ffap-url-p guess) (setq guess (substitute-in-file-name guess)))
;; Should not do it on url's, where $ is a common (VMS?) character.
;; Note: upcoming url.el package ought to handle this automatically.
guess))