via.json

{
  "menus": [
    {
      "label": "Layer",
      "content": [
        {
          "label": "Layer",
          "content": [
            {
              "label": "Layer",
              "type": "range",
              "options": [0, 8],
              "content": ["id_wl_layer", 0, 1]
            }
          ]
        }
      ]
    }
  ],
...

via_default_layer.c

enum  via_indicator_value {
    id_wl_layer = 1,
};

void wl_config_set_value(uint8_t *data) {
    // data = [ value_id, value_data ]
    uint8_t *value_id   = &(data[0]);
    uint8_t *value_data = &(data[1]);

    switch (*value_id) {
        case id_wl_layer:
            default_layer_set(1UL<<*value_data);
            break;
    }
}

void wl_config_get_value(uint8_t *data) {
    // data = [ value_id, value_data ]
    uint8_t *value_id   = &(data[0]);
    uint8_t *value_data = &(data[1]);

    switch (*value_id) {
        case id_wl_layer:
            *value_data = get_highest_layer(default_layer_state);
            break;
    }
}

void wl_config_save(void) {
    eeconfig_update_default_layer(default_layer_state);
}

void via_custom_value_command_kb(uint8_t *data, uint8_t length) {
    uint8_t *command_id        = &(data[0]);
    uint8_t *channel_id        = &(data[1]);
    uint8_t *value_id_and_data = &(data[2]);

    if (*channel_id == id_custom_channel) {
        switch (*command_id) {
            case id_custom_set_value: {
                wl_config_set_value(value_id_and_data);
                break;
            }
            case id_custom_get_value: {
                wl_config_get_value(value_id_and_data);
                break;
            }
            case id_custom_save: {
                wl_config_save();
                break;
            }
            default: {
                // Unhandled message.
                *command_id = id_unhandled;
                break;
            }
        }
        return;
    }
    *command_id = id_unhandled;
}