The recent news of an exploit for the help: URI handling on the Mac underscores how difficult it can be to change the MIME handlers under OS 10.X.
Internet Explorer (if you still have it installed) can manipulate them, but nothing in the OS interface lets you directly manipulate the mappings and handlers. Carl Lindberg has released a nice Pref Pane (RCDefaultApp) that lets you manipulate these settings as a control panel.