Can I add my own commands/scripts to the mail.app toolbar?
I think you can use Mail Act-on for this, it provides Shortcuts for doing stuff with various mails. Not a Visual Shortcut, but a Keyboard one.


  Move messages by keystroke only Mail
  Act-On’s interface includes the
  ability to move or copy messages to
  any folders by keystroke, even if you
  don’t have an Act-On rule for a
  particular mailbox.


http://www.indev.ca/MailActOn.html

Hope it helps, 

Karl