Actually I have a really cool idea to fix that. Let's say that you grant an AI civ right of passage, at the outpost you would be able to set whether the AI can actually pass through that outpost's area of influence.
The downer is that because what you're effectively doing is setting up a toll booth, there's no way that you can charge that in advance on the treaty. Because you could just revoke passage through a particular outpost and then it's a worthless treaty for the AI.
It means the AI has a choice, it can pay a modest fee for going through your checkpoint, getting just the access it needs to fight its enemies. They profit, you profit, until of course they decide they hate your guts and that toll booths are evil.