The Poisoned Chalice
explicitly establishes Magna Roma to be a Federation Member. From pages 98-99:
James Swallow, THE POISONED CHALICE wrote:
The other woman was the same age as Vale, from a planet commonly known as Magna Roma. Atia's people were the descendants of humans transplanted from Earth's distant past to that world by an alien race known as the Preservers. They had carried their culture--based on the Terran Roman Empire--with them and evolved a civilization based upon its tenets. The Magna Romanii had long since grown out of the more objectionable elements of their society and become active members of the Federation; but they were said to be a fierce, strident people, and Vale's first impression of her new executive officer did nothing to dispel that view.
Nice to see that James and I agreed on the Preserver origin. Makes so much more sense than Hodgkins' Stupid Law.