The way you enforce it is by signing and ratifying a treaty. Lets face it, the UN or international courts are all honour systems. The UN doesn't enforce jack against 1st world nations because they all have veto powers. Enforcement comes from political pragmatism and outmanoevering in public. You force one party or another to be accountable to their promises or conduct. See what I would find fascinating is if the way one house faction conducts itself in the generic inter-sirius body caused many other factions to react. Make RP politics into a slightly more organic living thing, as opposed to it just being staged and controlled by the party interested in it. There would be limits, but what if being a dick ont he intergalactic stage made everyone hate you? Maybe you'd not be so aggressive.
The other thing is it could follow the trends and currents of the current Mod story arc. Start with convening to discuss disputes with Kusari complaining about Bretonia's alliance with the Mollies who are mining up Tau-31. Then have Liberty and Rheinland bitch at each other sideways about sovereignty and the like since they're spoiling for a way. Make Liberty look aggressive and Rheinland defensive since they need to delay for time to prepare for whatever war might come, them being the weaker navy. You could have nations or systems represented by their owners. For instance OSC has a few systems, right in the middle of the war paths. OSC could be like a Poland caught between the USSR and Germany or a Belgium stuck between a France and Germany. It would be a great forum for not only exploring the limits of existing RP background but also practicing with them interacting with their natural counterparts and adversaries.
Lots of people don't like this idea. I'd go for it, as long as it was delegates speaking in an allowed order, like maybe a speaker of the house system, and it wasn't a free for all bitch fest.
EDIT. @Ace... really... I don't want to bitch you out. But honestly, you've seen the in game chat system. You can't organize factions and diplomacy in that. Its clumsy and limiting. You want to just run around and PvP all the time then go o the PvP server. Here we want a reason to have a PvP, and not just cause it'd be wicked sick to see crap blowd up real guuud. RP is why we're here, and its fun. Don't sh1t on our parade cause you don't have the patience for it.