For the terminal toolbar, it's just another XML file. Applications would have to claim to use it, so you can add it to your own application, but not platform wide (in hindsight, we should have added a global configuration for that, but yeah well …).

You can read the MeeGo terminal code, the toolbar registration happens here:
http://gitorious.org/meego-terminal/...ay.cpp#line435

Happy hacking!