I find that wanderlust regularly queries with "There's hidden deleted messages,
expunge anyway? (y or n)". To facilitate smoother interaction with
IMAP for those using multiple clients (which may handle delete/expunge
in varying ways), why not permit setting a default answer to the
query?

Perhaps something like

(defvar elmo-always-expunge-deleted t)

(luna-define-method elmo-folder-delete-messages-plugged
  ((folder elmo-imap4-folder) numbers)
  (let ((session (elmo-imap4-get-session folder))
        (expunge
         (or (null (elmo-imap4-list folder "deleted"))
             ;; check if default is set
             elmo-always-expunge-deleted
             (y-or-n-p
	      ;; polish grammar
              "There is a hidden deleted message; expunge anyway?")
             )))
    (elmo-imap4-session-select-mailbox
     session
     (elmo-imap4-folder-mailbox-internal folder))
    (unless (elmo-imap4-set-flag folder numbers "\\Deleted")
      (error "Failed to set deleted flag"))
    (when expunge
      (elmo-imap4-send-command session "expunge"))
    t))