Realm's on Atamara don't have Rulers, they have puppets. Why do you think CE's Senate has so many strings attached to the ends of their fingers?
My cynical bias against Atamara requires me to +1 this.
But, to the question at hand, I agree with Indirik. For one thing, I've been that Duke. Probably not as powerful, no, but let me put it to you this way - say you go to your Ruler with your super awesome ploy and he turns you down flat? What are you gonna do? Secede and completely blow up your realm? Rebel, and sort of blow up your realm? Sure you've got some leverage, but all the buttons you could push to really make your Ruler hurt involve hurting your realm too to some extent. Or a large extent, probably.
As another Ruler, a deal's not a deal until I hear it from a fellow Ruler, at least not a deal between realms. Anyone else and you don't know if they can back up what they're promising. If you think you had the leverage to get your ruler to agree to your plan, you should have had him contact your negotiating partners and state as much.