*scratch*.el

(defadvice magit-status(after enable-magit-svn-mode activate)
  "Enable `magit-svn-mode' when opening magit via `magit-status'
   and is a git svn repository."
  (require 'magit-svn)
  (when (magit-svn-get-ref-info)
    (turn-on-magit-svn)))

;; or..

(add-hook 'magit-mode-hook (lambda()
                             (require 'magit-svn)
                             (when (magit-svn-get-ref-info)
                               (turn-on-magit-svn))))