A new entry in the Size table needs to be made for 1200x600, so that Orbiter can access it. You can do this with any standard database tool, or even via command line with MySQL, look at the other rows in the Size table for examples.
As for Orbiter, you can copy the Orbiter binary to another linux machine. You'll also need a few dependencies:
* PlutoUtils (/usr/pluto/lib)
* SerializeClass (/usr/pluto/lib)
* SDL_Helpers (/usr/pluto/lib)
You can then run orbiter. If it detects this is a new orbiter, it will ask a few questions to set itself up. If you've created a new entry in the Size table, your resolution will show up in the list of choices.
Keep in mind, this orbiter is designed for dedicated use, so it will assume its rendering surface will completely cover the screen it is using.