Using memoization keys from previous step outputs

[MarcoLugo]

A few manifests to provide context:

This one works just fine but requires setting each output parameter individually:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
   generateName: dag-with-cache-
spec:
   entrypoint: start
   templates:
      - name: start
        dag:
           tasks:
              - name: generate-cache-keys
                template: generate-cache-keys
              - name: sleep
                dependencies: [generate-cache-keys]
                arguments:
                   parameters:
                      - name: cache-key
                        value: "{{tasks.generate-cache-keys.outputs.parameters.b}}"
                template: sleep
      - name: sleep
        inputs:
           parameters:
              - name: cache-key
        container:
           image: alpine:3.7
           command: [sh, -c]
           args: ["sleep 30s"]
        memoize:
           key: "{{inputs.parameters.cache-key}}"
           maxAge: "1m"
           cache:
              configMap:
                 name: cache-dag-test
      - name: generate-cache-keys
        script:
           image: alpine:3.7
           command: [bin/sh]
           source: |
              echo "abc" > /tmp/a
              echo "xyz" > /tmp/b
        outputs:
          parameters:
            - name: a
              valueFrom:
                path: /tmp/a
            - name: b
              valueFrom:
                path: /tmp/b

My actual workflow has dozens of of cache keys that are produced by a pod and having to set the output parameters individually is not only cumbersome but also makes it easier to forget to keep certain things in sync and so increases the possibility of error. My first take was maybe I can output the cache keys to a json file and then just have that json file as an output and finally parse that json to extract the key when required, something like:

apiVersion: argoproj.io/v1alpha1
kind: Workflow
metadata:
   generateName: dag-with-cache-json-
spec:
   entrypoint: start
   templates:
      - name: start
        dag:
           tasks:
              - name: generate-cache-keys
                template: generate-cache-keys
              - name: sleep
                dependencies: [generate-cache-keys]
                arguments:
                   parameters:
                      - name: cache-key
                        value: {{ fromJson(tasks.generate-cache-keys.outputs.parameters.cache-keys).b }}
                template: sleep
      - name: sleep
        inputs:
           parameters:
              - name: cache-key
        container:
           image: alpine:3.7
           command: [sh, -c]
           args: ["sleep 30s"]
        memoize:
           key: "{{inputs.parameters.cache-key}}"
           maxAge: "1m"
           cache:
              configMap:
                 name: cache-dag-test
      - name: generate-cache-keys
        script:
           image: alpine:3.7
           command: [bin/sh]
           source: |
              cat <<EOF > /tmp/outputs.json
              {
                "a": "abc",
                "b": "xyz"
              }
              EOF
        outputs:
          parameters:
            - name: cache-keys
              valueFrom:
                path: /tmp/outputs.json

This one does not work. I assume because expressions probably cannot be used in the memoize.key field. Is there a workaround for this or perhaps some other solution even if does not necessarily involve json? I do want to avoid having to create extra steps/nodes if possible.

Thank you for your help.