Would it be possible to mandate developers (by creating policies on in some other way) to mark all of their pods/resources with pre-defined label , e.g. environment=dev or department=dev ?
It can be in the form of enforcement ( e.g. if pod is created without propel label - deny the creation request) or just silently apply necessary label and re-write the existing one if needed.
I gave this answer to a similar question regarding "...policy check before container execution".
In essence, Open Policy Agent - a Cloud Native Computing Foundation (incubating) project - supports custom policy enforcement for Kubernetes. Check out this example regarding enforcing labels to be present.
It seems that Admission controller is the only way to accomplish it - https://kubernetes.io/blog/2019/03/21/a-guide-to-kubernetes-admission-controllers/