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"
                }
              ]
            }
          ]
        }
      ]
    }

Yep! You can do whatever you want with complex modifications. The only tricky part is figuring it out. It shouldn't be TOO different than the above if you replace the appropriate values.