(I don't have enough 'SO reputation' to leave a comment in the thread, so ill post my comment to frouhi here.)

Wonderful, I am glad to hear Iris-ZO is working well for you:) IRIS-ZO only requires an implementation of the CollisionChecker base class that determines if a configuration is in our out of collision. The a potential way to add nonlinear constraints would be to extend the implementation SceneGraphCollisionChecker to also accept nonlinear constraints (e.g. like so and so). In this extension one would have to treat constraint violations also as collisions.

I hope this helps for now!