I know that I can create a LimitRange in a namespace. Then all pods created in that namespace will have Resource (CPU/Memory) Limits/Requests set.
So my initial question of how to enforce Resource Limits/Requests on all resources in a kubernetes cluster seems to be equivalent to: How do I enforce a LimitRange exists in every namespace?
It is currently not possible to enforce LimitRange and ResourceQuota cluster-wide, that is, providing defaults for every namespace. I'm aware of at least one discussion to potentially change this.