ものすごい安易な解決だと、こんな感じで (or ...) でつなげば動くんじゃないの？試してないけど。

(defadvice navi2ch-article-get-message-list
(before navi2ch-article-get-message-list-ad activate)
(if (and navi2ch-article-message-filter-mode
(or (string-match "live" (cdr (assq 'uri navi2ch-article-current-board))))
(string-match "mnewsplus" (cdr (assq 'uri navi2ch-article-current-board)))))
(setq navi2ch-article-message-filter-mode nil)))