We found it pretty easy to build a little k8s controller for our own purposes to do this -- see https://news.ycombinator.com/item?id=48478994 . You probably don't need to implement this as a plugin or hook, pgdog supports dynamic reload of its configuration without dropping existing connections.
Although I'm the type to shy away from adding extra layers in my architectures when I can help it, pgdog has been an absolute breeze to use :)