If CR wants in on the treaty they would have to bring something to the table first. Yes CR patrols the system, and this is a good start. But currently the only thing I've seen from CR to OS&C is the message that a smuggler docked on Curacao.
To become a signatory I would assume you would need the support of a major party or at least multiple parties currently present in the treaty. Obviously IMG would be supportive, you say you have Bretonia. But before you start throwing around your weight it would always be nice to ask the groups.