How to flatten Google Analytics custom dimensions with a UDF in BigQuery?

Based on a post by Robert Sahlin, I want to use a BigQuery UDF to access any Google Analytics custom dimension in BigQuery by its index. In the proposed solution Robert uses a JavaScript UDF, and I'm wondering if it's possible to do the same with a SQL UDF - since a SQL UDF should perform better than a JS one.

The proposed JS UDF:

CREATE TEMPORARY FUNCTION customDimensionByIndex(index INT64, arr ARRAY<STRUCT<index INT64, value STRING>>)
RETURNS STRING
LANGUAGE js AS """
  for (var j = 0; j < arr.length; j++){
    if(arr[j].index == index){
      return arr[j].value;
    }
  }
  """;

SELECT 
  fullvisitorId,
  visitId,
  hit.hitnumber,
  customDimensionByIndex(6, hit.customDimensions) as author,
  customDimensionByIndex(7, hit.customDimensions) as category 
FROM `123456.ga_sessions_YYYYMMDD`
JOIN
  UNNEST(hits) as hit