Negotiations in IHW:SC can be modeled much like assaults, and this technique is portable across any system. You try to push the enemy out of the area he is holding, back to the place you want him. Each party - and there can be more than two - has a goal in the negotiations, and each side has a negotiating team. I like using an abstract map with the starting positions in the center and the various goals on the edges. Here's an example:
In the example, there are three parties negotiating a treaty dealing with an independent world's relation to SaVaHuTa. One side, A, is xenophobic, and wants the League out of the world entirely. Side B wants only trade with SaVaHuTa, with no political association. Side C wants full membership in the League.
Each side has one main negotiator, and an equal number of assisting, allied negotiator. The allies go first, each using a skill to manipulate the other side - the skill does not matter, although it needs to be explained as to how it can help. Intimidation, bribery, inspiration, conviction, seduction... anything is possibly of use. Each success at this level gives a Small bonus to the main negotiations, each failure giving a Small penalty. The main negotiators face off with their modifiers, using the appropriate skill. A failure means the side has to move from its position one area towards the winning side's goal.
If all sides win, then the negotiations remain deadloacked for another round - I like to make one round equal to a day, but it doesn't really matter. When a side has been pushed to another side's goal area, then that side has lost the negotiations and is out. The negotiations end after a set time period, for example 20 rounds, or only one side remains viable, whichever comes first. If time expires, the negotiations have failed and the status quo remains in force. If only one side remains viable, then the negotiations are successful, and that side wins.
I have adapted these rules from my own assault/boarding rules in previous incarnations of IHW, assisted and clarified by the Social Combat rules in Diaspora. You are - of course - free to roleplay it all out completely, to play this negotiation like a minigame, or mix and match roleplay and rules as you see fit.