Solved this but going to leave it up in case it's useful to anyone - just used the pod template in the values.yml for the clearml-agent helm chart to mount the hostpath as a volume mount i.e.:

    podTemplate:
    # -- volumes definition for pods spawned to consume ClearML Task (example in values.yaml comments)
    volumes:
      - name: x11-host-dir
        hostPath:
          path: /tmp/.X11-unix 
    volumeMounts:
      - name: x11-host-dir
        mountPath: '/tmp/.X11-unix'