An example of configuring aggregate cluster using EnvoyFilter. Currently needs patched Pilot with go-control-plane updated: gcr.io/howardjohn-istio/pilot:1575492376, and proxy from `master`

aggregate-cluster.yaml

# First define a basic deployment+service that will act as our local service
apiVersion: apps/v1
kind: Deployment
metadata:
  name: httpbin
  namespace: default
spec:
  selector:
    matchLabels:
      app: httpbin
  template:
    metadata:
      labels:
        app: httpbin
    spec:
      containers:
      - name: httpbin
        image: docker.io/kennethreitz/httpbin
        resources:
          limits:
            memory: "128Mi"
            cpu: "500m"
        ports:
        - containerPort: 80
---
apiVersion: v1
kind: Service
metadata:
  name: httpbin
  namespace: default
spec:
  selector:
    app: httpbin
  ports:
  - name: http
    port: 80
    targetPort: 80

# Next, we define a ServiceEntry for our external service that we will fall back to
---
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: fallback
  namespace: default
spec:
  hosts:
  - httpbin.internal #does not have to be resolvable
  location: MESH_INTERNAL
  ports:
  - number: 80
    name: http
    protocol: HTTP
  resolution: DNS
  endpoints:
  - address: httpbin.org
    locality: external
---

# Create a service entry just to get the proper routes set up.
# This is pretty much a hack
apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: mixed-mode-service
  namespace: default
spec:
  hosts:
   # has to be dns resolvable by app so that traffic exits app 
   # and reaches sidecar on port 80
  - httpbin.org
  location: MESH_INTERNAL
  ports:
  - number: 80
    name: http
    protocol: HTTP
  resolution: NONE
---

# Get the config patches set up
# We remove the original httpbin.org cluster with our new aggregate cluster
apiVersion: networking.istio.io/v1alpha3
kind: EnvoyFilter
metadata:
  name: aggregate-cluster
  namespace: default
spec:
  configPatches:
  - applyTo: CLUSTER
    match:
      cluster:
        name: "outbound|80||httpbin.org"
    patch:
      operation: REMOVE
  - applyTo: CLUSTER
    match:
      context: SIDECAR_OUTBOUND # will match outbound listeners in all sidecars
      cluster:
        name: "PassthroughCluster"
    patch:
      operation: ADD
      value:
        name: "outbound|80||httpbin.org"
        lb_policy: CLUSTER_PROVIDED
        connect_timeout: 1s
        cluster_type:
           name: envoy.clusters.aggregate
           typed_config:
            "@type": type.googleapis.com/envoy.config.cluster.aggregate.v2alpha.ClusterConfig
            clusters:
            - outbound|80||httpbin.default.svc.cluster.local
            - outbound|80||httpbin.internal
---
# outlierDetection is needed in order to determine when to fail over
apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: outlier
  namespace: default
spec:
  host: "httpbin.default.svc.cluster.local"
  trafficPolicy:
    outlierDetection:
      consecutiveErrors: 3

Because this was made over a year ago. No longer relevant

thanks for your super prompt response!