In ObjectStore we have a BindingContext (https://github.com/realm/realm-core/blob/master/src/realm/object-store/binding_context.hpp) which contains a number of important extension points.

This is e.g. used by the Java binding to create Realm change listeners for when anything in the Realm changes, something that is otherwise not possible to implement.

It would be nice to have it exposed in the C API as well, but I'm a bit unsure exactly how that would look like given the constraints we have there.