Does DragonDictate still work if you switch users? If so, you could encrypt your home directory, add a second user, and set OSX to auto-login to that user (and auto-launch DragonDictate).

If not, I think your best bet is a Bluetooth solution or some hardware token. For instance, an Arduino or Teensy ($20) programmed with your login/master password, and with a small microphone connected would be able to respond to certain voice commands and act as a regular USB keyboard, typing in your pass phrase.

You could also have some software on the Mac to automatically lock the screen or shutdown the computer if the Arduino is removed.