If we want to truly get the upper hand in further negotiations, just command Robb march towards King's Landing and siege it. THEN even Renly and Stannis will come to us for terms (i.e. "Nice you are doing this, but in the name of which King are you doing it, huh?").
Ned can't command Robb to do anything right now. He's still in enemy territory with no ability to communicate with the outside world.