My GUESS is that it would be something like this:

    {
      "title": "Hold z or / for control",
      "rules": [
        {
          "description": "Hold z for control",
          "manipulators": [
            {
              "type": "basic",
              "from": {
                "key_code": "z",
                "modifiers": {
                  "optional": [
                    "any"
                  ]
                }
              },
              "to_if_alone": [
                {
                  "key_code": "left_control"
                }
              ]
            }
          ]
        },
        {
          "description": "Hold / for control",
          "manipulators": [
            {
              "type": "basic",
              "from": {
                "key_code": "/",
                "modifiers": {
                  "optional": [
                    "any"
                  ]
                }
              },
              "to_if_alone": [
                {
                  "key_code": "right_control"
                }
              ]
            }
          ]
        }
      ]
    }